This is one of the webpages of Libarid A. Maljian at the Department of Physics at CSLA at NJIT.
New Jersey Institute of Technology
College of Science and Liberal Arts
Department of Physics
The Earth in Space
Phys 203
Fall 2025
Third Examination lecture notes
Introduction to Oceanology/Oceanography
The sum total of all water on
Earth is called the Earth’s hydrosphere. The study of the Earth’s hydrosphere is most commonly called oceanography, and persons who study
the Earth’s hydrosphere most commonly call themselves oceanographers. The study of the Earth’s hydrosphere should more properly be called oceanology, and persons who
study the Earth’s hydrosphere should more properly call themselves oceanologists.
Although more and more textbooks are renaming this subject oceanology
and although more and more persons who study this subject are calling
themselves oceanologists, most textbooks still
nevertheless refer to this subject as oceanography and most persons who study
this subject still nevertheless call themselves oceanographers. In this course, we will use the terms
oceanology and oceanography interchangeably, and we will use the words oceanologist and oceanographer interchangeably as
well. Roughly seventy percent of the world
is covered with oceans, while only thirty percent of
the world is covered with continents.
This is reason enough to devote a significant amount of time in this
course to oceanology/oceanography.
Moreover, most of the geologic activity of planet Earth is submarine
(underwater), simply because most of planet Earth is covered
with water. Most seismic activity such
as earthquakes occur at the bottom of the ocean. Most igneous activity such as lava eruptions
occur at the bottom of the ocean. Most
mountains are at the bottom of the ocean.
Even most landslides occur at the bottom of the ocean. For all of these reasons, we must devote a
significant amount of time in this course to oceanology/oceanography if we wish
to study our planet Earth.
The measurement of ocean
depths below sea level is called bathymetry, while the
measurement of continental elevations above sea level is called
hypsometry. Nearly everyone refers to
maps with contour lines of elevation as topographical maps, but these are more correctly called hypsometric maps. Note that the Greek root bathy- means deep,
while the Greek root hypso- means height. For thousands of years, bathymetric
measurements (ocean depths below sea level) were performed
using a technique called sounding. A
heavy object such as an anchor was lowered into the
ocean with a very long chain until the heavy object hit the seabed (the ocean
floor). We can be certain that the
anchor has hit the seabed if the chain becomes slack. Then the chain was
carefully pulled out of the ocean until the anchor was retrieved. The ocean depth was
concluded to be the length of the retrieved chain. As the chain was pulled
out of the ocean, sailors would count the number of times they pulled the chain
while their arms were outstretched. For
this reason, ocean depth has been and still continues
to be measured in units called fathoms.
The word fathom is derived from another word
meaning outstretched arms. However,
different sailors obviously have different arms of different lengths. Therefore, different sailors will measure a
different number of fathoms of ocean depth at the same location. Eventually, oceanographers/oceanologists realized that a fathom must
be redefined as a precise unit of ocean depth. The modern definition of the fathom is
precisely six feet (two yards) of ocean depth.
In other words, if we know the depth of the ocean in feet, we would
divide by six to determine the depth of the ocean in fathoms. If we know the depth of the ocean in yards,
we would divide by two to determine the depth of the ocean in fathoms. If we know the depth of the ocean in fathoms,
we would multiply by six to determine the depth of the ocean in feet or
multiply by two to determine the depth of the ocean in yards. Caution: it is incorrect to use fathoms for
any other measurement of length. We
would never ever express the height of a building or the elevation of a hill or
mountain or the width of a classroom or the height of a ceiling in fathoms. Only ocean depth is
measured in fathoms.
Beginning in the twentieth
century, bathymetric measurements began to be performed
using a technique called echo sounding.
A ship would use a speaker to emit a sound into the ocean. The sound would propagate to the seabed, and
the reflected sound wave would propagate back from the seabed. A reflected sound wave is
called an echo, which is why this technique is called echo
sounding. The ship would detect the echo
with a microphone. We know when the
sound was emitted, and we know when the echo was
detected. If we know the speed of sound
in water, we would use the equation speed equals distance divided by time to
determine the depth of the ocean.
Unfortunately, the speed of sound in the ocean depends upon many
variables such as temperature, pressure, and even salt content. To roughly estimate
the ocean depth, we can use the average speed of sound in the ocean, which is
roughly fifteen hundred meters per second or almost one mile per second. As a simple example, suppose we emit a sound
in the ocean, and suppose we detect the echo four seconds later. If the average speed of sound in the ocean is
roughly one mile per second and if we detect the echo four seconds later, then
the depth of the ocean is roughly two miles.
Caution: we must remember to take half of the time because it takes half
of the time for the sound to propagate downward to the seabed and it takes the
other half of the time for the echo to propagate upward back to the ship. In the previous example, half of four seconds
is two seconds, making the ocean depth two miles if the average speed of sound
in the ocean is roughly one mile per second.
If a whale happens to be swimming under the ship when we emit a sound
into the ocean, we may detect the echo from the whale, and our calculation will
grossly underestimate the ocean depth.
Although it is improbable that a whale just happens to be swimming under
the ship when we emit a sound into the ocean, even a school of fish swimming
under the ship will cause an echo that the ship will detect. Again, our calculation will grossly
underestimate the ocean depth.
Therefore, a more accurate form of echo sounding was devised called
SONAR, which is an acronym for SOund Navigation And Ranging. With
this technique, a ship emits many sounds into the ocean, and the ship detects
many echoes. Then, a computer is used to analyze these data to yield an accurate
calculation for the depth of the ocean.
After decades of bathymetric
measurements, we have concluded that ocean depths are much deeper below sea
level than continental elevations above sea level. The maximum ocean depth below sea level is
roughly eleven kilometers, but the maximum elevation of the continents above
sea level is less than nine kilometers.
The maximum elevation of the continents above sea level is Mount Everest
in the Himalayas in Asia, while the maximum ocean depth below sea level is
Challenger Deep in the Mariana Trench in the Pacific Ocean. Admittedly, this is a comparison of extreme
values above and below sea level, but a comparison of average values above and
below sea level reveals that the difference is even more
extreme. The average ocean depth
below sea level is roughly 3.7 kilometers, but the average elevation of the
continents above sea level is roughly 840 meters, less than one kilometer! In other words, the average depth of the
ocean is between four and five times deeper below sea level than the elevation
of the continents above sea level.
Unfortunately, even after decades of bathymetric measurements, only
roughly five percent of the ocean floor has been mapped
in detail. There are still large areas
of the ocean floor that have not been mapped at
all. Sadly, we know the topography of
other worlds such as Venus and Mars and the Earth’s Moon more accurately than
we know the topography of our own ocean floor.
Geological Oceanology/Oceanography
As we discussed earlier in
the course, research conducted by the United States Navy beginning immediately
after the end of the Second World War led directly to the formulation of the
Theory of Plate Tectonics, the fundamental theory of the geology of our planet
Earth. The United States Navy continues
to this day to explore the ocean floor.
For example, the Deep Sea Drilling Project (DSDP)
begun in 1968 drilled and retrieved rocks from the ocean floor. The Deep Sea Drilling Project (DSDP) was eventually replaced by
the Ocean Drilling Program (ODP) begun in 1983, which
could not only drill rocks at the ocean floor but could drill more than two
kilometers deeper than the ocean floor!
The Ocean Drilling Program (ODP) used the ship
the JOIDES Resolution to drill submarine rocks. JOIDES is an
acronym for Joint Oceanographic Institutions for Deep Earth Sampling. On this ship, a large drill bit would be
lowered into the ocean, and a piece would be attached
to that drill bit. Both were lowered further into the ocean, and another piece would
be attached. As the entire drill was lowered, more and more pieces were attached until the
entire drill was long enough to reach the ocean floor. Further pieces could be
attached to drill more than two kilometers deeper than the seabed. After drilling was completed, the entire
drill was slowly lifted while piece by piece was
removed until the drill bit was returned to the ship. In brief, the entire drill was
assembled and disassembled wherever the JOIDES
Resolution planned to drill submarine rocks. The Ocean Drilling Program
(ODP) was replaced by the Integrated Ocean Drilling
Program (IODP) in 2003. This mission was renamed the International
Ocean Discovery Program (IODP) in 2013, which is
still in operation. This mission is able
to drill more than seven kilometers deeper than the ocean floor!
The continental margin is the
edge of the continent, where the continent meets the ocean. Note that the word margin means edge. All continental margins can
be divided into two categories: active continental margins and passive
continental margins. There is an
abundance of geologic activity at active continental margins, such as igneous
activity and seismic activity. This geologic activity is caused by either a transform fault or a
subduction zone at the active continental margin. There is very little geologic activity at
passive continental margins. Examples of
passive continental margins include the east coast of North America, the east
coast of South America, and the west coast of Africa. Examples of active continental margins
include the west coast of North America and the west coast of South America. When studying oceanology/oceanography, we use
the term trench for subduction zones. As
we discussed earlier in the course, there are two different types of subduction
zones: oceanic-oceanic subduction and oceanic-continental subduction. If the term trench is another word for the
term subduction zone and if there are two different types of subduction zones,
then there are two different types of trenches.
An oceanic-oceanic subduction zone is called an
oceanic trench, while an oceanic-continental subduction zone is called a
continental trench. Caution: both types
of trenches are at the bottom of the ocean, even though one of these types is called a continental trench. In summary, we find continental trenches and
transform faults at active continental margins.
The general topography of a passive
continental margin is as follows. First,
the continent extends out into the ocean, and this extension of the continent
into the ocean is called the continental shelf. We emphasize that the continental shelf is a
part of the continent; the continental shelf is the part of the continent that
happens to be submerged under the ocean. The continental shelf is composed of the same
felsic igneous rock that composes the rest of the continent. This is strong evidence that the continental
shelf is indeed a part of the continent.
If sea levels happened to be higher, more of the continent would be submerged, making the continental shelf even
larger. If sea levels happened to be
lower, less of the continent would be submerged,
making the continental shelf smaller.
There is an abundance of ocean life on the continental shelf since
sunlight is able to penetrate into these shallow waters, as we will
discuss. Beyond the continental shelf,
the seabed steepens to deeper depths below sea level. This is called the
continental slope, and beyond the continental slope where the seabed begins to
level out is called the continental rise.
Beyond the continental rise is the ocean basin, also called the abyssal
plain. The ocean basins (abyssal plains)
are the largest, flattest parts of the surface of the entire world. Caution: it is a common misconception that
only the continents comprise the surface of the Earth. Actually, the continents together with the
seabed comprise the surface of the Earth.
The seabed just happens to be the part of the surface of the Earth that is submerged under the ocean waters. In fact, most of the surface of the Earth is
the seabed. The ocean basins (abyssal
plains) are the largest, flattest parts of the surface of the world, even
though they are submerged at the bottom of the
ocean. The edge of the ocean basin
(abyssal plain) is the edge of the tectonic plate where we find oceanic ridges,
which are mountain ranges built from submarine igneous activity, as we discussed
earlier in the course.
There are submarine canyons
within the continental slope. These
submarine canyons are not carved out by rivers from
the continental interior. As we will
discuss later in the course, rivers are bodies of running water on the
continents that carve out channels for themselves on the continents. Nevertheless, river waters do not continue to
carve into the rock when arriving at the ocean.
If submarine canyons were carved out by rivers,
then every submarine canyon would be located where the mouth of a river meets
the ocean, but this is not the case.
Moreover, submarine canyons would be built from
the continent toward the ocean if they were carved out by rivers. In fact, the opposite is true: the shape of
submarine canyons reveals that they are built from the
ocean toward the continent. Submarine
canyons are actually carved out by turbidity currents,
which are submarine landslides. Gravity
pulls rocks and sediments downward, causing a turbidity current. These turbidity currents carve submarine
canyons into the continental slope. A
turbidity current is actually a mixture of rocks, sediments, mud, and water
pulled downward by gravity. This is why
they are called turbidity currents; the adjective
turbid means murky or thick. The
adjective murky may also mean mentally unclear, and hence the adjective turbid may also mean perplexed or confused. Note that the words turbulent and turbulence are related to the adjective turbid, since these words are
used to describe anything that is disordered and hence unpredictable. The first historical evidence that submarine
canyons are caused by turbidity currents was the
disruption of transatlantic telegraph cables in the early twentieth
century. Before humans used mobile
telephones for rapid communication, they used landline telephones. Before humans used landline telephones for
rapid communication, they used telegraphs.
The telegraph was invented in the 1830s
by American inventor Samuel Morse, for whom the Morse code was named. Telegraph cables were laid
down between many of the major cities of the United States during the 1850s. These
telegraphs played an important role in the economic Panic of 1857 as well as
the American Civil War from 1861 to 1865.
Giant telegraph cables were also dropped to the bottom of the Atlantic
Ocean during the 1850s, permitting fairly
rapid communication between the United States and Europe. For centuries, it took several weeks for a
ship to cross the Atlantic Ocean.
Therefore, anything that occurred in the United States
would not be known by Europeans until weeks later, and anything that
occurred in Europe would not be known by Americans until weeks later. After these transatlantic telegraph cables were dropped to the bottom of the Atlantic Ocean,
information was relatively rapidly communicated across thousands of kilometers
of ocean for the first time in human history.
The Grand Banks earthquake in the year 1929 caused a turbidity current
that not only carved out a submarine canyon but also disrupted several
transatlantic telegraph cables. The
discovery of these damaged telegraph cables revealed that turbidity currents
carve out submarine canyons in the continental slope.
The continental rise is also built by turbidity currents. Each time a turbidity current occurs, rocks
and sediments slide down and land at the base of the continental slope. After many such turbidity currents, the
result is an accumulation (a pile) of rocks and sediments at the base of the
continental slope. This is the
continental rise. Each of these accumulations of rocks and sediments that compose the
continental rise is called a deep sea fan or a submarine fan. Similar processes occur on the continents, as
we will discuss later in the course.
Whenever a continental landslide occurs, rocks and sediments slide down
and land at the base of the hill or mountain.
After many such landslides, the result is an accumulation (a pile) of
rocks and sediments at the base of the hill or mountain.
The ocean basins or abyssal
plains are the largest, flattest parts of the surface of the world. These ocean basins (abyssal plains) are also covered with thousands of mountains. Nearest the oceanic ridge are seamounts, the
tallest mountains in the ocean. Roughly
halfway from the oceanic ridges to the continents we find medium-height mountains called tablemounts or guyots. These
medium-height mountains are called tablemounts
because they often have flat tops like a table.
This is also why they are called guyots. The American
geophysicist Harry Hammond Hess, one of the fathers of the Theory of Plate
Tectonics, discovered and studied these tablemounts
and called them guyots because they resemble Guyot Hall, a flat-roofed building at Princeton University
in New Jersey named for the Swiss-American geographer Arnold Henry Guyot. Closest to
the continent we find the shortest mountains called abyssal hills or seaknolls. These
seamounts, tablemounts (guyots),
and abyssal hills (seaknolls) provide strong evidence
for the Theory of Plate Tectonics.
Molten rock at divergent plate boundaries cools and crystallizes to form
seamounts nearest the oceanic ridges.
Over millions of years, the tectonic plate moves while natural forces
degrade the seamounts, making them shorter and shorter until they have become tablemounts (guyots) roughly halfway
from the oceanic ridges to the continents.
Over many more millions of years, the tectonic plate continues to move
while natural forces continue to degrade the tablemounts
(guyots), making them shorter and shorter until they
have become abyssal hills (seaknolls). Furthermore, the rock that composes the
seamounts is youngest, the rock that composes the tablemounts
(guyots) is middle-aged, and the rock that composes
the abyssal hills (seaknolls) is oldest, all
consistent with the predictions of the Theory of Plate Tectonics. This continuum of rock ages
was discovered by the seafloor-drilling programs of the United States Navy,
as we discussed earlier in the course.
As we discussed, the ocean
basins (abyssal plains) are the largest, flattest parts of the surface of the
world. As we also discussed, the ocean
basins (abyssal plains) are covered with thousands of
mountains. Are we not contradicting
ourselves? How can an ocean basin
(abyssal plain) be flat while at the same time be covered
with mountains? The resolution of this
paradox is as follows. All of these
mountains on the ocean basins (abyssal plains) are themselves covered with
sediments. Since the rock closest to the
continents is oldest, there has been more time for sediments
to accumulate, resulting in thicker sediment.
Since the rock closest to the oceanic ridges is youngest, there has been
less time for sediments to accumulate, resulting in thinner sediment. The thickest sediment covers the shortest
abyssal hills (seaknolls), while the thinnest
sediment covers the tallest seamounts.
Hence, the entire ocean basin (abyssal plain) levels out to a fairly flat topography.
This continuum of sediment thicknesses was discovered
by the seafloor-drilling programs of the United States Navy, as we discussed
earlier in the course.
The sedimentary rock covering
ocean basins (abyssal plains) is primarily biogenic sedimentary rock, and the
composition of this biogenic sedimentary rock closely matches the composition
of the lifeforms living near the surface of the ocean directly on top of the
rock. At first glance, this would seem
to be logical. When marine life dies,
their carcasses should sink to the bottom of the ocean, and layer upon layer
upon layer of carcasses should accumulate and eventually lithify forming
biogenic sedimentary rock. However, this
argument is too simplistic. Perhaps this
argument is correct for the largest fish or whales when they die, but most fish
can be easily carried by ocean currents a far distance from the seabed rocks that
share their composition. The microscopic
marine lifeforms that live near the surface of the ocean can certainly be
carried a far distance by ocean currents.
Nevertheless, the seabed rocks share their composition as well. Part of the resolution of this paradox is
that small fish eat these microorganisms.
Then, medium fish eat the small fish.
Large fish then eat the medium fish.
Whales may eat the microorganisms.
Eventually, the large fish and the whales die, and their carcasses sink
to the bottom of the ocean, accumulate, and eventually lithify to form these
biogenic sedimentary rocks. Although
this argument correctly accounts for some of the formation of the biogenic
sedimentary rock at the ocean floor, this argument does not explain the
formation of most of this rock. The
primary resolution of this paradox is that most ocean life excrete fecal
pellets that are dense enough to sink to the ocean floor. Layer upon layer upon layer of this excrement
accumulates on the seabed and eventually lithifies to form biogenic sedimentary
rock. The vulgar word for feces or
excrement is sh!#$&*t. To summarize, if we were able to walk upon
the ocean floor, we would be walking upon biogenic sedimentary rock that has
been lithified from layer upon layer upon layer of heavy, hard, dense sh!#$&*t.
Chemical Oceanology/Oceanography and Thermodynamic Oceanology/Oceanography
The water molecule is
composed of one oxygen atom covalently bonded to two hydrogen atoms. The oxygen atom shares one pair of electrons
with one of the hydrogen atoms, and the oxygen atom shares another pair of
electrons with the other hydrogen atom.
However, the oxygen atom also has two other pairs of electrons that it
does not share with the hydrogen atoms.
The shape of a molecule while ignoring all unshared pairs of electrons
is called its molecular geometry, but the shape of a molecule while including
all unshared pairs of electrons is called its electronic geometry. The electronic geometry (including the two
unshared pairs) of the water molecule is a distorted tetrahedron. Since the two unshared pairs of electrons are
on the oxygen side of the water molecule, this side of the water molecule
carries a significant negative charge.
The other side of the water molecule where the two hydrogen atoms reside
carries a significant positive charge.
Of course, the entire water molecule is neutral overall. We conclude that the water molecule is
electrically dipolar. A molecule is
electrically dipolar if the entire molecule is neutral overall but one side of
the molecule is positively charged while the other
side of the molecule is negatively charged.
The water molecule is quite strongly electrically dipolar, and this
property of the water molecule gives rise to several extraordinary properties
of water. For example, water molecules
can attract and repel other water molecules.
As we discussed toward the beginning of the course, like electrical
charges repel, and unlike electrical charges attract. Thus, if two water molecules have their
hydrogen sides facing each other, they will repel each other, since positive
and positive repel. Similarly, if two
water molecules have their oxygen sides facing each other, they will also repel
each other, since negative and negative repel.
However, if the hydrogen side of one water molecule is facing the oxygen
side of another water molecule, they will attract each other since positive and
negative attract. The ability of water
molecules to attract other water molecules is called
cohesion. The surface
tension of water is caused by water molecules cohesively attracting each other. Water molecules can even attract other
molecules besides water; this is called adhesion. Adhesion together with cohesion is
responsible for the meniscus of water within a test tube. The water molecules adhesively attract the
glass or plastic composing the test tube and then cohesively attract other
water molecules to form the meniscus.
Adhesion together with cohesion is also responsible for the leaves of
trees obtaining water. The leaves of a
tree do not obtain water directly from rain.
Instead, rainwater first seeps into the ground. This water adhesively sticks to the tree
roots. Then, this water cohesively
attracts other water molecules within the soil.
All these water molecules are adhesively attracted
further up the tree root, and they cohesively attract other water molecules
with them. Gradually, the water defies
gravity by moving up the tree roots, up the tree trunk, up the tree branches,
and eventually to the leaves of the tree.
This process by which water is transported
upward within trees is called capillary action, which would not occur if water
were not cohesive and adhesive due to the water molecule being strongly
electrically dipolar.
If we spread chemicals upon a
table, they will not react. Molecules
must be within a liquid so that they can float within the liquid and meet each
other so that they will react. However,
chemicals will still not react if they are free to float and meet each other
within most liquids. The liquid must
dissociate (rip apart) the molecules so that they will be able to react with
each other. As a concrete example,
suppose we wish for sodium chloride NaCl to react
with magnesium sulfate MgSO4 to yield
sodium sulfate Na2SO4 and
magnesium chloride MgCl2. This properly balanced reaction is written 2 NaCl + MgSO4
Na2SO4 + MgCl2. If we pour
sodium chloride NaCl and magnesium sulfate MgSO4 upon a table, they will not react. If we immerse sodium chloride NaCl and magnesium sulfate MgSO4
within a liquid where they are free to float and find each other, they may
still not react. The liquid must
dissociate (rip apart) the sodium chloride NaCl
molecules into sodium cations Na1+
and chlorine anions Cl1–, and
the liquid must also dissociate (rip apart) the magnesium sulfate MgSO4 molecules into magnesium cations Mg2+ and sulfate anions SO42–. These dissociations are necessary so that the
sodium cations Na1+ and the
sulfate anions SO42–
are both free to bond with each other to form sodium sulfate Na2SO4 molecules and so that the
magnesium cations Mg2+ and the
chlorine anions Cl1– are both
free to bond with each other to form magnesium chloride MgCl2
molecules. We conclude that in order for
a chemical reaction to occur, the molecules must not only be within a liquid so
that they are free to float and find each other, but the liquid must also
dissociate (rip apart) the molecules so that chemical bonding can occur. All molecules we wish to react are collectively called the solute, the liquid they are
immersed within that will dissociate them is called the solvent, and the solute
together with the solvent is called the solution. In the previous example, we may be able to
find a liquid that is a good solvent for sodium chloride NaCl,
but that liquid may not be a good solvent for magnesium sulfate MgSO4.
Perhaps we can find another liquid that is a good solvent for magnesium
sulfate MgSO4, but this other liquid may
not be a good solvent for sodium chloride NaCl. Perhaps we may be fortunate enough to find a
liquid that is a good solvent for both sodium chloride NaCl
and magnesium sulfate MgSO4, but that
liquid may not serve as a good solvent for other solutes. The water molecule is so strongly
electrically dipolar that nearly any solute we immerse into water will be dissociated (ripped apart). This is because the oxygen side (the negative
side) of the water molecule attacks one side of almost any molecule, while the
hydrogen side (the positive side) of the water molecule attacks the other side
of almost any molecule. This results in the molecular dissociation of nearly every
molecule immersed within water. For this
reason, chemists consider water to be the universal solvent, and hence nearly
all chemical reactions will proceed within water. Although there are some molecules where not
even water serves as a good solvent since water does not dissociate those
molecules, such cases are rare. The
water molecule is so strongly electrically dipolar that nearly any chemical
reaction will occur within water.
A lifeform is anything that
is born, that grows and develops, that is able to move, that ingests substances
necessary for its survival, that excretes substances
harmful to its survival, that is able to reproduce, and that must eventually
die. All of these actions of life
require energy. Every second of every
day of our lives, we are expending energy.
Even while asleep, our brains continue to expend energy keeping our
hearts beating and our lungs breathing, which themselves expend energy as they
function. Death is when a lifeform
ceases expending energy. All lifeforms
derive the continuous energy that they must expend to remain alive from
chemical reactions, but chemical reactions must occur within a solvent. We now realize why all lifeforms are composed
of mostly water. The human body is
mostly water for example. All life
continuously expends energy, and all lifeforms derive their energy from
chemical reactions. Chemical reactions
must occur within a solvent, and water is the universal solvent. Therefore, all lifeforms must be composed of
mostly water. Imagine a plastic bag
filled with water with some reacting chemicals immersed within the water. This is what a lifeform is essentially,
including humans. A human is essentially
a plastic bag of water with some reacting chemicals immersed within the
water. This also explains why life began
within the oceans and why for most of the history of planet Earth all life was
in the oceans. To the present day, most
of the biomass of our planet is within the oceans, as we will discuss.
The heat capacity of any
substance is the amount of heat that we must add to make it hotter by a certain
temperature. The heat capacity is also
the amount of heat that we must extract from the substance to make it colder by
a certain temperature. More plainly, the
heat capacity of any substance is how difficult it is to change its
temperature. If we must add much heat to
make something hotter or if we must extract much heat to make something colder,
this means that it is difficult to change its temperature. If we must add only a small amount of heat to
make something hotter or if we must extract only a small amount of heat to make
something colder, this means that it is easy to change its temperature. The strong cohesion of water (the strong
attractions between water molecules) causes water to have a large heat
capacity. That is, we must add much heat
to water to make it hotter, and we must extract much heat from water to make it
colder. In other words, it is difficult
to change the temperature of water. More
plainly, hot water tends to remain hot, and cold water tends to remain
cold. This is one reason why humans have
consumed soup for thousands of years.
Hot water tends to remain hot.
Thus, food placed within hot water will remain hot for a longer duration
of time. We can demonstrate this with a
simple experiment. Consider two
identical bowls of hot chicken soup for example. Suppose we scoop all of the broth out of one
of the bowls, and suppose we wait for only a couple minutes. The chicken that remained in the broth-empty
bowl has already become rather cold, while the chicken that remained within the
broth-full bowl is still quite hot. Our
planet Earth is mostly covered with ocean. Since water has a large heat capacity, it is
difficult to change the temperature of water.
Therefore, all of the water that covers planet Earth acts to stabilize
temperatures throughout the entire planet.
For example, all of the water that covers planet Earth moderates the
temperature difference between daytime and nighttime. The daytime side of any planet faces toward
the Sun, while nighttime side of any planet faces away from the Sun. For most planets, nighttime is much colder
than daytime, but the nighttime side of planet Earth is only slightly cooler
than its daytime side, thanks to the stabilizing effect of all the water that
covers most of the planet.
Extraordinarily, the nighttime side of planet Earth may at times become
warmer than the daytime side depending upon weather patterns, as we will
discuss later in the course. As another
illustration of how water stabilizes temperatures, desert nighttimes
are significantly colder than desert daytimes because of a scarcity of water,
which would have moderated the extreme difference in temperature between
nighttime and daytime. As another
example of how water stabilizes temperatures, other planets have north poles
and south poles that are much colder than their equators. Although the Earth’s poles are cold and the
Earth’s equator is hot by human standards, the difference in temperature is
nevertheless moderate as compared with other planets. Without the abundance of water that covers
our planet Earth, our poles would be too cold and our equator would be too hot
to be habitable for life. Water currents
within the ocean also contribute to the moderation of temperatures between the
poles and the equator, as we will discuss.
As yet another example of how water stabilizes
temperatures, continental coasts enjoy mild winters and mild summers due to the
nearby ocean, where again the abundance of water stabilizes temperature
differences. This is
called the marine effect. In
other words, the marine effect causes both mild summers and mild winters near
the ocean as compared with severely hot summers and severely cold winters far
from the ocean in the middle of continents, which is called
the continental effect. That is, the
continental effect causes severe seasons (both hot summers and cold winters)
far from the ocean. We can extend this
logic globally, across the entire planet Earth.
The southern hemisphere is covered with much
more ocean as compared with the northern hemisphere. In fact, the southern hemisphere’s nickname
is the water hemisphere, while the northern hemisphere’s nickname is the land
hemisphere. Since the southern
hemisphere is the water hemisphere, temperatures are more
stabilized in the southern hemisphere, thus causing more mild
seasons. Since the northern hemisphere
is the land hemisphere, temperatures are more extreme
in the northern hemisphere, thus causing more severe seasons. In other words, if we compare two cities, one
in the northern hemisphere and one in the southern hemisphere, we expect the
northern-hemisphere city to suffer from hotter summers and colder winters,
while we expect the southern-hemisphere city to enjoy mild summers and mild
winters. This assumes that all other
circumstances are equal between the two cities, such as their degrees of latitude
from the equator, their location on their respective continents relative to
oceans, and the location of nearby mountain ranges and lakes. Often, even exceptions to this general
difference between the two hemispheres can be explained
with the marine effect versus the continental effect. Although both the North Pole and the South
Pole are cold throughout the entire year and although northern hemisphere
winters are generally colder than southern hemisphere winters, it is in fact the South Pole that is colder than the North
Pole. At the North Pole, there happens
to be an ocean, the Arctic Ocean, and hence the marine effect moderates the
cold temperatures, making the North Pole less cold. At the South Pole, there happens to be a
continent, Antarctica, and hence the continental effect does not moderate the
cold temperatures, making the South Pole more cold. The cold temperatures at the South Pole are further intensified by an ocean current that surrounds
and hence isolates Antarctica from the rest of the world, as we will discuss.
The oceans are not pure
water. There are various different salts
dissolved within the ocean. The most
abundant salt in the Earth’s ocean is sodium chloride NaCl,
which is ordinary table salt. Since
water is the universal solvent, the salts dissolved within the ocean are dissociated into positive ions (cations) and negative
ions (anions). Therefore, it is more
correct to say that the most abundant salt in the Earth’s ocean is sodium
chloride NaCl which is
dissociated into sodium cations Na1+
and chlorine anions Cl1–. The salt in the ocean comes from three
sources: rivers from continents, turbidity currents, and volcanic
activity. As rivers flow on the
continent, the running water degrades the continental rock, ultimately breaking
the rock down into sediments and minerals.
Among these minerals are salts.
The running water ultimately delivers these salts to the ocean. Turbidity currents (submarine landslides)
liberate trapped rocks, sediments, and minerals within the continental slope
into the ocean, and among these liberated minerals are salts. Whenever any igneous eruption occurs, gases,
rocks, sediments, and minerals are ejected from the
eruption in addition to lava, as we discussed earlier in the course. Some of these ejected minerals are
salts. Most igneous activity is
submarine, as we discussed. Therefore,
whenever a submarine volcanic eruption occurs, salts are
sprayed into the ocean. Even
continental igneous eruptions add salt to the ocean. An igneous eruption on a continent will eject
salts into the atmosphere that may drift for hundreds of kilometers before
finally landing on the ocean. Since all
of these mechanisms continuously add salt to the ocean, we might incorrectly
conclude that the ocean thus becomes saltier and saltier. This is not the case; the overall saltiness
of the Earth’s oceans remains roughly constant.
If the overall saltiness of the Earth’s oceans remains roughly constant,
then salt must be subtracted from the ocean at the
same overall rate that salt is added to the ocean. Salt is removed from
the ocean by both sedimentation and biological processes. Firstly, the salt in the ocean slowly sinks
to the seabed, eventually lithifying to become sedimentary rock. Secondly, lifeforms in the ocean extract
salts and other minerals from the ocean for their own purposes, such as making
hard shells for example. In summary,
salt is added to the ocean at the same overall rate
that salt is subtracted from the ocean.
Hence, the overall saltiness of the ocean remains roughly constant.
The technical term for the
saltiness of water is the salinity of the water. More precisely, the salinity of water is defined as the amount of salt in the water as a fraction
of the total amount of water. That is,
salinity is directly related to the amount of salt and inversely related to the
amount of water. The first half of this
definition is obvious. If salinity is directly related to the amount of salt, then greater salt
increases the salinity and lesser salt decreases the salinity. It is almost a waste of time to try the
following experiment, since the result is easy to predict. Suppose we taste the saltiness of a cup of
salt water. If we add more salt and if
we taste it again, it will taste more salty.
If we scoop out some of the salt and if we taste it yet again, it will
taste less salty. However, the second
half of the definition of salinity is less obvious. If salinity is also
inversely related to the amount of water, then greater water decreases
the salinity and lesser water increases the salinity. This can be demonstrated
with the following experiment. Suppose
we taste the saltiness of a cup of salt water.
If we add more water and if we taste it again, it will taste less salty. Some students argue that this is obvious,
since adding water dilutes the amount of salt in the cup, thus making it less
salty. This is essentially correct, but
it is still remarkable. We have not
subtracted any salt from the cup; we have only added water. Nevertheless, we have succeeded in decreasing
the salinity, without subtracting any salt!
Conversely, suppose we taste the saltiness of a cup of salt water, and
suppose we leave the cup of salt water upon the kitchen counter for a few
hours, thus permitting some of the water to evaporate. If we taste it again, it will taste more
salty. Some students argue that this is
obvious, since evaporation subtracts water, thus concentrating the amount of
salt in the cup making it more salty.
This is essentially correct, but it is still remarkable. We have not added any salt to the cup; we
have only subtracted water.
Nevertheless, we have succeeded in increasing the salinity, without
adding any salt! To summarize, salinity is directly related to the amount of salt, and salinity is
inversely related to the amount of water.
We emphasize again the second half of this definition of salinity. It is possible to change the salinity of
water without changing the amount of salt but instead by changing the amount of
water. In fact, this is the primary way
the salinity of the ocean water changes: by the addition or subtraction of
water, not primarily by the addition or subtraction of salt.
Let us review the basic
mathematics of fractions. Consider the
fraction four-sevenths, often written 4/7.
What is the meaning of this fraction?
What is the meaning of the four, and what is the meaning of the
seven? Most students respond that the
four is the numerator while the seven is the denominator. These terms numerator and denominator are
just words. Students who regurgitate
such terms actually reveal a lack of understanding of fractions. What is the meaning of the numerator, and
what is the meaning of the denominator?
The denominator of any fraction is the number of equal pieces of the
whole or total, and the numerator of a fraction is the part of that whole or
total. In other words, a fraction is a
part of a whole or a part of a total.
This is the meaning of the fraction four-sevenths for example. How would a student eat four-sevenths of a
pizza pie? First, we divide the whole
pizza pie into seven equal slices (pieces).
Then, the student would eat four of those seven slices. A percent is a particular type of fraction
where the whole or the total is always divided into
one hundred equal pieces. This is why it
is called percent.
The Latin root cent- always means one hundred. There are one hundred cents in a dollar. There are one hundred centimeters in a
meter. There are one hundred years in a
century. There are many other words with
this Latin root cent- meaning one hundred.
The percent symbol is written %. For example, consider two percent, written
2%. The meaning of two percent is two
parts out of a whole or total of one hundred equal
pieces. This means that two percent is
actually two-hundredths, which could be written 2/100. The whole or total is divided into one
hundred equal pieces (as always with percent), and we take two of those one
hundred equal pieces.
Some parts of the Earth’s
oceans have greater salinity (are more salty), while other parts of the Earth’s
oceans have lesser salinity (are less salty).
The average salinity of the Earth’s oceans is the total amount of salt
in the ocean as a fraction of the total amount of water in the ocean. This is an averaging of all the different
salinities of all the different parts of the ocean. This average salinity of the Earth’s oceans
is roughly 3.5%. In other words, for
every one hundred pounds we pull out of the ocean, 3.5 pounds will be salt and
the remaining 96.5 pounds will be water.
Note that this is on average; again, some parts of the Earth’s ocean
have greater salinity, while other parts of the Earth’s ocean have lesser
salinity. In science, we always use the
metric system. Although our calculation
using pounds is technically correct, we should instead use metric units such as
kilograms. So,
for every one hundred kilograms we pull out of the ocean, 3.5 kilograms will be
salt and the remaining 96.5 kilograms will be water. Again, this is on average.
The denominator of a fraction
need not be one hundred. We can choose
the denominator of a fraction to be one thousand for example. The symbol % is
commonly regarded as the percent symbol, but it is also called the
parts-per-hundred symbol. For example,
2% is most often read two percent, but 2% may also be
read two parts-per-hundred. The new
symbol ‰ is called the parts-per-thousand symbol. This symbol looks like the parts-per-hundred
(percent) symbol except for an extra zero in the denominator. As an example of using this new symbol, 9‰ is
read nine parts-per-thousand, meaning 9/1000 or nine-thousandths or nine out of
a whole or total of one thousand equal pieces.
How do we convert from parts-per-hundred (percent) to
parts-per-thousand? We will use 2% as a
concrete example. First, 2% equals
2/100, but we are free to multiply the numerator and the denominator of this
fraction by ten yielding 20/1000, which is 20‰.
Thus, 2% becomes 20‰ by simply multiplying two by ten to yield
twenty. In conclusion, we convert
parts-per-hundred (percent) to parts-per-thousand by multiplying by ten. This stands to reason: two out of one hundred
is the same part of the whole as twenty out of one thousand. We can demonstrate this with the following
concrete example. Suppose there are one
hundred persons who each have ten dollars.
The total amount of money is one thousand dollars, since one hundred persons times ten dollars each equals one thousand
dollars. If we select two of these
persons, we have also selected twenty dollars, since two persons
times ten dollars each equals twenty dollars.
Notice that when we selected two persons out of one hundred persons, we
have also selected twenty dollars out of one thousand dollars. This makes it clear that two out of one
hundred is the same fraction (the same part of the whole) as twenty out of one
thousand.
In oceanology/oceanography,
we almost always measure salinity in
parts-per-thousand, for reasons we will make clear shortly. The average salinity of the ocean is 3.5%,
which we now realize is 35‰, since 3.5 multiplied by ten is thirty-five. In other words, for every one thousand pounds
we pull out of the ocean, thirty-five pounds will be salt and the remaining 965
pounds will be water. This is on average
of course. Although our calculation
using pounds is correct, we should instead use metric units such as
kilograms. For every one thousand
kilograms we pull out of the ocean, thirty-five kilograms will be salt and the
remaining 965 kilograms will be water.
Again, this is on average. We almost always measure salinity in parts-per-thousand in
oceanology/oceanography because there are one thousand grams in one
kilogram. Therefore, for every one
kilogram (one thousand grams) we pull out of the ocean, thirty-five grams will
be salt and the remaining 965 grams will be water, again on average.
Some parts of the ocean have
a salinity greater than 35‰, other parts of the ocean have a salinity less than
35‰, and still other parts of the ocean have a salinity roughly equal to
35‰. Whereas both the addition and
subtraction of salt and the addition and subtraction of water play roles in the
varying salinity of the ocean, the addition or subtraction of water much more
commonly determines the salinity of the ocean.
Although ocean water with a salinity greater than 35‰ could result from
the addition of salt, it much more commonly occurs through the subtraction of
water. Although ocean water with a
salinity less than 35‰ could result from the subtraction of salt, it much more
commonly occurs through the addition of water.
Water that has a salinity significantly greater than 35‰ is called hypersaline water.
The Greek root hyper- means over or too much, and the adjective saline
describes or modifies anything related to salt.
Water that has a salinity significantly less than 35‰ is
called brackish water. The word
brackish means salty-ish or not as salty as
expected. Water that has a salinity
roughly equal to 35‰ is called seawater. Since the addition or subtraction of water is
the more important variable determining salinity in the ocean, we can draw
general conclusions about these different types of water. We expect brackish water in the ocean where
there is greater precipitation (rain or snow from the atmosphere) than
evaporation from the ocean. We expect
hypersaline water in the ocean where there is greater evaporation from the
ocean than precipitation (rain or snow from the atmosphere). Finally, we expect seawater where
precipitation and evaporation are roughly in balance.
Water on the continents such
as in lakes and in rivers and running out of our faucets is
commonly known as freshwater, but this is a misleading word. The term freshwater implies that there are no
salts within the water, but there are always salts within water. The salt could be such a tiny amount that we
do not taste the salts, but the salinity is nevertheless not zero. The proper term for water on the continents
is limnological water. The study of water on the continents is called limnology, commonly regarded as the study of
freshwater using this misleading term. Someone
who studies water on the continents is called a limnologist,
commonly regarded as someone who studies freshwater, again using this
misleading term. Note that the Greek
root limno- means freshwater lake.
We measure salinity with a
device called a salinometer, which operates based
upon the principle of electrical conduction.
Salt within water makes the entire solution a good electrical conductor,
and the electrical conductivity of the solution is directly
related to the salinity. In other
words, greater salinity results in greater electric conductivity, and lesser
salinity results in lesser electric conductivity. To build a primitive salinometer,
we apply a fixed voltage to a sample of water, and we measure the electric
current flowing through the solution. If
we measure a greater electric current, then the electric conductivity must be
greater, which reveals that the salinity must be greater. If we measure a lesser electric current, then
the electric conductivity must be less, which reveals that the salinity must be
less. It is a common misconception that supposedly the same applied voltage always results in the
same electric current, but this is false.
The electric current is determined not only by the
applied voltage but also by the electric conductivity of the
material. In other words, if we apply a
certain voltage across one object with a lesser electric conductivity, that
voltage will push less electric current through that object. However, if we apply that same voltage across
another object with a greater electric conductivity, that same voltage will
push more electric current through that second object. To measure the salinity of ocean water, a
ship lowers two electrodes into the ocean and always applies the same voltage
across those two electrodes, regardless where the ship is located in the
ocean. At some locations, the ship will
measure greater electric current in the water between the electrodes, which
implies the electric conductivity is greater and therefore the ocean water has
greater salinity. At other locations,
the ship will measure less electric current in the water between the
electrodes, which implies the electric conductivity is less and therefore the
ocean water has less salinity. Another
common misconception is that water is itself a good electric conductor. This is false; pure water is in fact a poor
electric conductor. However, the salts
within water do make the entire solution a good electric conductor. Since there are always some salts in water,
this makes the entire solution a good electric conductor. Never ever use an electric appliance or even
touch a light switch while wet. We may
not taste salts within the water running out of our faucets, but the salinity
is nevertheless not zero. Even tiny
amounts of salt within water results in a good electric conductivity.
The deep ocean water mixes
with itself very well, for reasons that we will make clear shortly. If the deep ocean water mixes with itself
very well, then its salinity must average out to roughly 35‰. If the surface ocean water is hypersaline
water, then its salinity is greater than 35‰.
If we descend into the deep ocean, the salinity must therefore decrease
to 35‰. If the surface ocean water is
brackish water, then its salinity is less than 35‰. If we descend into the deep ocean, the
salinity must therefore increase to 35‰.
Whether the surface ocean water is hypersaline water or brackish water,
we conclude that the salinity must steeply change as we descend into deep ocean
water. This steep change in salinity is called the halocline.
The Greek root -cline means steeply changing, and the Greek root halo-
means salt. If the surface ocean water
is hypersaline water, we must have a decreasing halocline as we descend into
the deep ocean water. If the surface
water is brackish water, we must have an increasing halocline as we descend
into the deep ocean water. If the
surface water is seawater, its salinity is roughly equal to 35‰. As we descend into the deep ocean, the
salinity must therefore remain roughly equal to 35‰. Therefore, there is no halocline if the
surface water is seawater. These
circumstances are called isohaline. The Greek root iso-
means not changing or steady or constant, the opposite of the Greek root
-cline. Again, the Greek root halo-
means salt. If the surface ocean water
is seawater, then there is no halocline as we descend into the deep ocean. The salinity remains isohaline.
Again, the deep ocean water
mixes with itself very well, for reasons that we will make clear shortly. If the deep ocean water mixes with itself
very well, then its temperature must be roughly uniform just as its salinity is
roughly uniform. The temperature of the
deep water is cold, roughly equal to the freezing temperature of water. This is because sunlight cannot penetrate
into the deep water, as we will discuss shortly. At the equator, the surface water is warm,
since the equator is hot throughout the year.
If we descend into the deep ocean, the temperature must therefore
decrease to roughly the freezing temperature.
We conclude that the temperature must steeply change as we descend into
deep ocean water. This steep change in
temperature is called the thermocline. Again, the Greek root -cline means steeply
changing, and the Greek root thermo- means temperature in words such as
thermometer, endothermic, and ectothermic.
At the equator, we must have a decreasing thermocline. At the poles, the surface ocean water is cold,
roughly equal to the freezing temperature of water, since the poles are cold
throughout the year. As we descend into
the deep ocean, the temperature must therefore remain roughly equal to the
freezing temperature of water.
Therefore, there is no thermocline.
These circumstances are called isothermal. Again, the Greek root iso-
means not changing or steady or constant, the opposite of the Greek root
–cline, and the Greek root thermo- means temperature. There is no thermocline at the poles; the
temperature remains isothermal as we descend into the deep ocean. At the midlatitudes,
the temperature of the surface ocean water depends upon the season. It is hot in the summertime, and it is cold
in the wintertime. In the summertime,
the surface ocean water is warm, but the deep ocean water is still cold. Therefore, we must have a decreasing
thermocline in the summertime, just as at the equator throughout the year. In the wintertime, the surface ocean water is
cold, but the deep ocean water is also cold.
Therefore, we do not have a thermocline; the temperature is isothermal
in the wintertime, just as at the poles throughout the year.
The salinity and the
temperature together determine the density of water. The density of water is
directly related to the salinity, and the density of water is inversely
related to the temperature. Since the
density is directly related to the salinity, a greater
salinity results in more dense water, and a lesser salinity results in less
dense water. This stands to reason. We expect a greater quantity of salt to make
the water heavier, and we expect a lesser quantity of salt to make the water
lighter. However, the density of water
is also inversely related to the temperature; hot water is less dense, and cold
water is more dense.
If hot water is less dense, it will be buoyed upward
by the surrounding water.
Therefore, hot water must rise.
This is true for any liquid as well as any gas such as air. In other words, this is true for any fluid:
hot fluids rise because they are less dense.
If cold water is more dense, it will be pulled
downward into the surrounding water.
Therefore, cold water must sink.
This is also true for any liquid as well as any gas such as air. In other words, this is true for any fluid:
cold fluids sink because they are more dense. Some ocean waters have a greater density,
while other ocean waters have a lesser density.
Whereas both salinity and temperature play roles in the varying density
of ocean waters, temperature is the more important variable. Although ocean water that is more dense could result from a greater salinity, it more
commonly occurs because of colder temperatures.
Although ocean water that is less dense could result from a lesser
salinity, it more commonly occurs because of warmer temperatures. Since the temperature is the more important
variable that determines density, we can deduce the density of ocean water from
its temperature. At the equator, the
surface ocean water is warm; therefore, it is less dense. The deep ocean water is cold; therefore, it
is more dense.
We conclude that the density must steeply change as we descend into the
deep ocean water. This steep change in
density is called the pycnocline. Again, the Greek root -cline means steeply
changing, and the Greek root pycno- means
density. At the equator, we must have an
increasing pycnocline. At the poles, the surface ocean water is
cold, but the deep ocean water is also cold.
There is no thermocline (the temperature is isothermal); therefore,
there is no pycnocline. These circumstances are
called isopycnal. Again, the Greek root iso-
means not changing or steady or constant, the opposite of the Greek root
-cline. Again, the Greek root pycno- means density.
There is no pycnocline at the poles; the
density remains isopycnal as we descend into the deep
ocean. At the midlatitudes,
the temperature of the surface ocean water depends upon the season; therefore,
the density must also depend upon the season.
It is hot in the summertime, and it is cold in the wintertime. In the summertime, the surface ocean water is
warm, but the deep ocean water is still cold.
Therefore, the surface ocean water is less dense, and the deep ocean
water is more dense.
Therefore, we must have an increasing pycnocline
in the summertime, just as at the equator throughout the year. In the wintertime, the surface ocean water is
cold, but the deep ocean water is also cold.
Therefore, we do not have a thermocline; the temperature is isothermal
in the wintertime, just as at the poles throughout the year. Thus, we do not have a pycnocline;
the density is isopycnal in the wintertime, just as
at the poles throughout the year. The pycnocline at the equator throughout the year and at the midlatitudes in the summertime is actually a cutoff between
the surface ocean water and the deep ocean water. If the surface ocean water is hot, then it is
less dense and hence it must rise. If
the deep ocean water is cold, then it is more dense
and hence it must sink. Notice therefore
that the surface ocean water and the deep ocean water are not able to mix with
each other. If the density at the poles
throughout the year and at the midlatitudes in the
wintertime is isopycnal, then there is no pycnocline and thus there is no cutoff between the surface
ocean water and the deep ocean water. In
other words, the surface ocean water and the deep ocean water mix well with
each other when there is no pycnocline, when the
density of the ocean water is isopycnal.
Biological Oceanology/Oceanography
Sunlight is only able to
penetrate well into the uppermost layer of the ocean. This uppermost layer is
called the euphotic zone. The
word euphotic literally means good light, since the Greek root photo- means
light in words such as photograph, photographer, and photon for example and the
Greek root eu- means good or well in words such as
euphoria or euphemism for example. The
euphotic zone only reaches depths of roughly one hundred meters below sea
level. Deeper than the euphotic zone is
the disphotic zone, where there is less light than
the euphotic zone. The word disphotic literally means bad light, since again the Greek root photo- again means light and the Latin
root dis- means bad in words such as disgrace, dislike, or disintegrate for
example. The disphotic
zone only reaches depths of roughly one kilometer below sea level. Actually, there is a fair amount of light at
the top of the disphotic zone, which gradually
transitions to poor light at the bottom of the disphotic
zone. For this reason, the disphotic zone is also called the
twilight zone. Deeper than the disphotic zone is the aphotic zone, where there is no
light. The word aphotic literally means
no light, since again the Greek root photo- again means light and the Greek
root a- means no or not in words such as apathy, asynchronous, and asymmetrical
for example. Most of the ocean is the
aphotic zone, since it extends from the bottom of the disphotic
zone all the way down to the ocean floor.
As we discussed, all life was
in the oceans for most of the history of planet Earth. To the present day, most of the biomass of
planet Earth is in the oceans, meaning that the total amount of marine
(oceanic) life weighs much more than the total amount of terrestrial
(continental) life. All marine life can be divided into three categories: the plankton, the
nekton, and the benthos. The plankton
are microscopic marine life that cannot generate their own locomotion; they
cannot swim. The plankton can only float
or drift in the ocean. Note that the
Greek root plank- means drifting. The
nekton are marine life that can generate their own locomotion; they can
swim. Fish and whales
and dolphins are classified as nekton.
Note that the Greek root nek- means
swimming. The benthos are the marine
life that live at the seafloor, such as clams, lobsters, and crabs. Note that the Greek root bathy- means
deep. Some textbooks claim that a human
who happens to be within the ocean (at the beach for example) is now a form of
marine life. They are after all a
lifeform in the ocean! By this
interpretation, a human swimming in the ocean becomes a nekton. Also by this interpretation, a human floating
in the ocean becomes a plankton; this is a rather large plankton! We will not adopt this extreme
interpretation. The plankton can themselves be subdivided into four subcategories: the
phytoplankton, the zooplankton, the bacterioplankton,
and the virioplankton. The bacterioplankton
are obviously bacteria floating in the ocean, and the virioplankton
are obviously viruses floating in the ocean.
We will concentrate our discussion on the phytoplankton and the
zooplankton. The phytoplankton use the
energy of sunlight to synthesize their own food using photosynthesis. Again, the Greek root photo- means light. In other words, phytoplankton are essentially
microscopic plants floating in the ocean, since terrestrial (continental)
plants also use the energy of sunlight to synthesize their own food using
photosynthesis. Zooplankton are
essentially microscopic animals floating in the ocean. The Greek root zoo- means animal. For example, zoology is the study of animals,
a zoologist is someone who studies animals, and we visit animals at the
zoo. Some species of zooplankton eat
various species of phytoplankton. These zooplankton are analogous to terrestrial herbivorous
animals that eat terrestrial plants. Other species of zooplankton eat various other
species of zooplankton. These zooplankton are analogous to terrestrial carnivorous
animals that eat other terrestrial animals.
Terrestrial plants use the
energy of sunlight to synthesize their own food using photosynthesis. An herbivorous animal may eat those
plants. A carnivorous animal may then
eat that herbivorous animal. Perhaps
another carnivorous animal eats that carnivorous animal, and so on and so forth
until we arrive at a carnivorous animal that no other animal eats, such as a
lion or a tiger for example. This is the
terrestrial food chain. The carnivorous
animal that no other animal eats is said to be at the
top of the food chain, while plants are said to be at the bottom of the food
chain. Any animal closer to the top of
the food chain is said to be higher on the food chain,
while any animal closer to the bottom of the food chain is said to be lower on
the food chain. The use of the term food
chain implies that if one animal becomes extinct, then all other animals higher
on the food chain will starve and hence also become
extinct. In general, this is not the
case. A carnivorous animal may eat a
variety of different animals for example.
If a particular animal becomes extinct, most of the animals higher on
the food chain will adapt their diets by eating other animals instead. For this reason, the term food web is more
correct than the term food chain.
However, notice that if all terrestrial plants were to become extinct,
then all herbivorous animals would starve, the carnivorous animals that eat
those herbivorous animals would starve, the carnivorous animals that eat those
carnivorous animals would starve, and so on and so forth all the way up to the
top of the food web. Hence, if all terrestrial
plants were to become extinct, all the rest of terrestrial life would become
extinct as well. Instead of saying that
plants are at the bottom of the terrestrial food web, it is more correct to say
that plants are the foundation of the terrestrial food web. The marine (oceanic) food web operates
similarly. We begin with phytoplankton
that use the energy of sunlight to synthesize their own food using
photosynthesis. Some species of
zooplankton may eat those phytoplankton. Another species of zooplankton may then eat
that zooplankton. Perhaps a small fish
eats that zooplankton. Perhaps a medium
fish eats that small fish. Perhaps a large
fish eats that medium fish, and so on and so forth until we arrive at an animal
that no other animal eats, such as a shark for example. This is the marine food chain. The animal that no other animal eats is said to be at the top of the food chain, while the
phytoplankton are said to be at the bottom of the food chain. Any animal closer to the top of the food chain
is said to be higher on the food chain, while any
animal closer to the bottom of the food chain is said to be lower on the food
chain. It is usually many steps from the
bottom of the marine food chain to the top of the marine food chain, but not
always. For example, whales eat
plankton. This is an example of jumping
from the bottom of the food chain all the way to the top of the food chain in a
single step. Again, the use of the term
food chain implies that if one animal becomes extinct, then all other animals
higher on the food chain will starve and hence also
become extinct. In general, this is not
the case. A fish may eat a variety of
other fish for example. If a particular
fish becomes extinct, most of the fish higher on the food chain will adapt
their diets by eating other fish instead.
For this reason, the term food web is more correct than the term food
chain. However, notice that if all
phytoplankton were to become extinct, then the zooplankton that eat
phytoplankton would starve, the zooplankton that eat those zooplankton would
starve, the small fish that eat those zooplankton would starve, and so on and
so forth all the way up to the top of the food web. Hence, if all phytoplankton were to become extinct,
all the rest of marine life would become extinct as well. Even various species of aquatic birds would
suffer from such an extinction of marine life.
Instead of saying that phytoplankton are at the bottom of the marine
food web, it is more correct to say that phytoplankton are the foundation of
the marine food web.
Anything whatsoever that
occurs in the biosphere is an example of biological productivity, such as one
organism eating another organism or an organism reproducing and so on and so
forth. Even the death of an organism
contributes to biological productivity. There are two fundamental variables that determine
biological productivity. The first is
the availability of sunlight. If there
is abundant sunlight, we expect the phytoplankton to use that abundance of
sunlight to synthesize an abundance of food causing them to proliferate. The zooplankton that eat phytoplankton will
also proliferate. The zooplankton that eat
those zooplankton will also proliferate. The small fish that eat those
zooplankton will also proliferate, and so on and so forth. Thus, biological productivity at all levels
of the food web should be high. However,
if there is little sunlight, the phytoplankton do not have sufficient energy to
synthesize an abundance of food, and they will be scarce in number. The zooplankton that eat phytoplankton will
also be scarce. The zooplankton that eat
those zooplankton will also be scarce. The small fish that eat those
zooplankton will also be scarce, and so on and so forth. Thus, biological productivity at all levels
of the food web should be low. However,
the other variable that determines biological productivity is the availability
of nutrients. Phytoplankton may have all
the sunlight they wish, but phytoplankton cannot synthesize food out of thin
air. The phytoplankton require nutrients
as the raw materials to synthesize their food, just as terrestrial plants
cannot create food out of thin air even if they had an abundance of sunlight. Terrestrial plants need soil from which they
extract the minerals they use as the raw materials to synthesize their
food. These two
variables that determine biological productivity, availability of sunlight and
availability of nutrients, are equally important. Without nutrients to synthesize food at the
foundation of the food web, an abundance of sunlight is insufficient to drive
biological productivity. Without
sunlight to provide the energy to synthesize food at the foundation of the food
web, an abundance of nutrients is insufficient to drive biological
productivity.
We might conjecture that
biological productivity is high in the equatorial oceans, since there is an
abundance of sunlight. Although this
abundance of sunlight should make biological productivity high, this abundance
of sunlight causes something else that makes biological productivity low: a
thermocline that causes a pycnocline, as we
discussed. The sunlight warms the
surface water making it less dense. The
sunlight cannot penetrate into the deep water, and therefore the deep water is
cold, making it more dense. The less dense surface water is buoyed upward, while the more dense deep water is pulled
downward. Hence, the resulting pycnocline is actually a cutoff between the surface water
and the deep water. Most marine life
lives near the surface of the ocean.
After all, the phytoplankton must live in the euphotic zone to access
sunlight. It follows that the
zooplankton that eat phytoplankton must also live in the euphotic zone, the zooplankton that eat those zooplankton must also
live in the euphotic zone, and so on and so forth. Fish that are able to swim into deep water
must occasionally swim to the surface to eat, and aquatic mammals such as
dolphins and whales that are able to swim into deep water must occasionally swim
to the surface to not only eat but to breathe air as well. Since the phytoplankton in the euphotic zone are cut off from the deep water by the pycnocline,
they cannot access the tremendous abundance of nutrients that fills the entire
ocean. Even though there is an abundance
of sunlight at the equatorial oceans, the biological productivity is low due to
a lack of nutrients. The biological
productivity is so low in the equatorial oceans that oceanologists/oceanographers
consider them to be marine deserts, analogous to the terrestrial deserts where
there is a scarcity of water and hence a scarcity of life.
There is insufficient
sunlight at the polar oceans. Although
this insufficient sunlight should make biological productivity low, this
insufficient sunlight causes something else that should make biological
productivity high: isothermal water and hence isopycnal
water, as we discussed. The insufficient
sunlight keeps surface water cold. The
sunlight cannot penetrate into the deep water, and therefore the deep water is also cold. There
is no thermocline, and thus there is no pycnocline. There is no cutoff between the surface water
and the deep water. Therefore, there is
good mixing between surface water and deep water. Thus, the phytoplankton in the euphotic zone are not cut off from the deep water, and they are able to
access the tremendous abundance of nutrients that fills the entire ocean. Nevertheless, there is insufficient sunlight
at the polar oceans, making the biological productivity low, even though there
is an abundance of nutrients.
Notice that availability of
sunlight and availability of nutrients are inversely related
to one another: abundant sunlight causes insufficient nutrients, and
insufficient sunlight causes an abundance of nutrients! We are tempted to sadly
conclude that nowhere in the oceans is biological productivity high, but
we have yet to analyze the midlatitude oceans. During wintertime at the midlatitude
oceans, there is insufficient sunlight, which should make biological
productivity low, but this insufficient sunlight causes something else that
should make biological productivity high: isothermal water and hence isopycnal water.
There is no cutoff between the surface water and the deep water. Therefore, there is good mixing between
surface water and deep water. Thus, the
phytoplankton in the euphotic zone are not cut off
from the deep water, and they are able to access the tremendous abundance of
nutrients that fills the entire ocean.
Nevertheless, there is insufficient sunlight, making the biological
productivity low even though there is an abundance of nutrients. However, springtime causes a miracle. There is more and more sunlight, which should
make biological productivity high.
Moreover, water has a large heat capacity, meaning that it is difficult
to change the temperature of water.
Thus, the surface water is not yet warm.
In other words, the thermocline has not yet formed, and thus the pycnocline has not yet formed. The surface water is still
not cut off from the deep water.
There is still good mixing between surface water and deep water, and
hence there is still an abundance of nutrients.
We finally have both abundant sunlight and abundant nutrients; hence,
the biological productivity is high.
Terrestrial biological productivity is similar. There is low biological productivity in the
wintertime, which is called hibernation. Terrestrial life comes out of hibernation in
the springtime, which is called the spring bloom. We will use these same terms for marine
life. Both equatorial marine life and
polar marine life hibernate throughout the year, although for completely
opposite reasons. Equatorial marine life
hibernates throughout the year due to insufficient nutrients, while polar
marine life hibernates throughout the year due to insufficient sunlight. Midlatitude marine
life hibernates in wintertime and enjoys a spring bloom in springtime. Whereas terrestrial life and midlatitude marine life both hibernate in wintertime and
both enjoy a spring bloom in springtime, their biological productivities become
opposite to each other in summertime and autumntime. In summertime, there is even more sunlight
than springtime, which should make biological productivity high, but this
abundance of sunlight causes something else that makes biological productivity
low: the surface water has now warmed while the deep water remains cold. The thermocline is
established, which causes a pycnocline that is
a cutoff between the surface water and the deep water. Thus, there is a lack of nutrients, and the
biological productivity is low. Midlatitude marine life goes back into hibernation in
summertime! This is quite different from
terrestrial life. Terrestrial life
hibernates in wintertime and then enjoys a spring bloom; terrestrial life
continues to enjoy high biological productivity in summertime, and then
biological productivity decreases in autumntime until
terrestrial life hibernates again in wintertime. Midlatitude ocean
life hibernates in wintertime and then enjoys a spring bloom; this is the same
as terrestrial life. However, midlatitude marine life goes back into hibernation in
summertime; this is different from terrestrial life. In autumntime, we
have another difference between terrestrial life and midlatitude
marine life. Although there is less and
less sunlight, there is still more sunlight as compared with wintertime. This decreasing sunlight causes the surface
water to cool. This destroys the
thermocline, which destroys the pycnocline. There is more and more mixing between the
surface water and the deep water. Thus,
the availability of nutrients becomes greater and greater. With more and more nutrients and with a
satisfactory abundance of sunlight, the biological productivity is high. This is called the
autumn bloom. Midlatitude
marine life returns to hibernation in wintertime because of a scarcity of
sunlight even though there is an even greater abundance of nutrients. To summarize, midlatitude
marine life goes through the following annual cycle: low biological
productivity in wintertime (hibernation), high biological productivity in
springtime (spring bloom), low biological productivity again in summertime
(hibernation), and high biological productivity again in autumntime
(autumn bloom) before returning to wintertime hibernation. The majority of the biomass of planet Earth
is marine (ocean life). Therefore, we
may assert that the biosphere of planet Earth hibernates in winter and summer
and blooms in spring and autumn.
Although only midlatitude marine life follows
this annual cycle, the terrestrial life comprises only a small part of the
biomass of planet Earth. Note that there
are exceptions to this analysis even for marine life, as we will discuss.
A coral reef is a vast
community of marine life living in close proximity with one another. It is obvious how new organisms join a coral
reef: the organisms simply join the community that has
already been established.
However, how does the coral reef begin in the first place? It is improbable for a large number of marine
organisms to come together at the same place at the same time to form a
community. The theory
of the formation of a coral reef was formulated by the British scientist
Charles Darwin, who also formulated the theory of biological evolution. First, marine organisms attach themselves to
a seamount; this is called a fringing coral reef. More and more organisms join this small
community until the fringing coral reef becomes a barrier coral reef. As more and more organisms join the barrier coral
reef, it eventually becomes a coral atoll.
If this theory is correct, then we should find a tablemount (guyot) in the deep
waters beneath a coral atoll, since the seamount that the fringing coral reef
attached itself to in the first place would have been degraded by natural
forces over time. This is indeed
the case. In the deep waters beneath
coral atolls, we find tablemounts (guyots), which were formerly the seamounts the marine life
attached themselves to when the fringing coral reef first formed.
Hydrothermal vents are
sources of geothermal energy at the ocean floor. There are entire communities of organisms
living around these hydrothermal vents.
The foundation of the food web of these communities cannot be
photosynthesis, since most of the ocean is the aphotic zone where it is
completely dark. Some of the organisms
in these communities are actually using the Earth’s geothermal energy to
synthesize their own food; this is called
chemosynthesis. Other organisms in these
communities may eat those organisms, yet other organisms in these communities
may eat those organisms, and so on and so forth. This is the food web of
these communities that live around hydrothermal vents at the ocean floor. The foundation of the food web for these
communities is chemosynthesis. This is
remarkable; biologists formerly believed that all life required sunlight,
either directly or indirectly. Even a
carnivorous animal requires sunlight at least indirectly,
since such an animal eats another animal lower on the food web which itself
eats another animal even lower on the food web that eats another animal which
eventually eats plants that require sunlight for photosynthesis. Nevertheless, we now realize that all life does not require sunlight. Whereas photosynthesis serves as the
foundation of the food web for most of the biosphere of planet Earth, it is chemosynthesis that serves as the foundation of the
food web for the communities at the ocean floor living around hydrothermal
vents. To our knowledge, all life absolutely
requires two things: liquid water (for reasons we have already discussed) and a
source of energy. The Sun is the source
of energy for most life in the biosphere on planet Earth, but even the Sun is
not the source of energy for all life on planet Earth. On our own planet Earth, geothermal energy
serves as the source of energy for some of the lifeforms of our planet’s
biosphere.
Physical Oceanology/Oceanography: Ocean Currents
Ocean currents are flowing
bodies of water in the ocean, rather like rivers are
flowing bodies of water on the continents.
We divide ocean currents into two categories: surface currents and deep
currents. Surface currents flow
horizontally, along the surface of the ocean.
Deep currents flow vertically, within the deep ocean. Surface currents are caused
primarily by the prevailing winds, while deep currents are caused by density
variations of ocean waters. We have
tremendous knowledge about surface currents, and so we will discuss them
extensively. Not much is
known about deep currents, and so we will discuss them only briefly.
The velocity (both the speed
and the direction) of surface currents is measured
with a device called a drift meter, which is essentially a plastic bottle. To measure the velocity (both the speed and
the direction) of a surface current, we drop a drift meter into the ocean at
one location, and we retrieve the drift meter from a different location
somewhere else in the ocean. We know
where we dropped and where we retrieved the drift meter. We also know when we dropped and when we
retrieved the drift meter. Using the
equation speed equals distance divided by time, we can
calculate the speed of the surface current that carried the drift meter. We can even determine the direction of the
surface current, since we know where we retrieved the drift meter relative to
where we dropped the drift meter.
Unfortunately, if we drop one drift meter into the ocean, we will most
likely never find it again. If we drop
ten drift meters into the ocean, we will most probably never find even one of
them. We predict the same unfortunate
outcome if we drop one hundred drift meters into the ocean. If we drop one thousand drift meters into the
ocean, perhaps we will retrieve one of them, but this would provide insufficient
data to perform a reliable calculation.
If we drop ten thousand drift meters into the ocean, perhaps we will
retrieve ten of them, but this still provides insufficient data to perform a
reliable calculation. If we drop one
hundred thousand drift meters into the ocean, perhaps we will retrieve one
hundred of them which would provide sufficient data to
perform a reliable calculation, but this would also be too expensive. Amusingly, among the most accurate
measurements of the motion of surface currents resulted from accidentally
spilling objects into the ocean. For
example, in May 1990 almost sixty-two thousand Nike sneakers were
accidentally dropped into the North Pacific Ocean. As these Nike sneakers washed up along the
shore of the west coast of North America, oceanologists/oceanographers
were able to reliably calculate surface currents in
the North Pacific Ocean. In the 1980s, Nike sneakers were a huge fad or craze; people
actually murdered each other to steal the Nike sneakers that they were wearing. Because of this fad or craze, people
immediately snatched up these Nike sneakers that
washed up on the west coast of North America.
Few persons would be lucky enough to find a matching pair. Without an internet to find the match to an
unmatched sneaker, people attended flea markets to either
pay top dollar if they found the match to their sneaker or to trade one
unmatched sneaker for another matching sneaker.
In January 1992, one hundred thousand plastic bathtub toys (such as
rubber duckies) were accidentally dropped into the
North Pacific Ocean, again enabling oceanologists/oceanographers
to reliably calculate surface currents in the North Pacific Ocean. The largest such accidental spill in history
was the February 1997 dropping of roughly half a million Lego pieces into the
North Atlantic Ocean. This enabled oceanologists/oceanographers to reliably
calculate surface currents in the North Atlantic Ocean. In March 2001, more than a quarter of a
million plastic soap dispensers were accidentally dropped
in the North Atlantic Ocean, again enabling oceanologists/oceanographers
to reliably calculate surface currents in the North Atlantic Ocean. Although these accurate measurements of
surface currents have occurred only in recent decades, we have had knowledge of
surface currents stretching back roughly five hundred years, at least in the
North Atlantic Ocean, as we will discuss shortly.
Surface currents are caused primarily by the prevailing winds. We will discuss the prevailing winds in
detail later in the course. For now,
winds generally blow in certain particular directions. We are not saying that winds always blow in
certain particular directions; we are saying that winds blow much more often in
certain particular directions as compared with other directions. These predominant directions are called the prevailing winds. These prevailing winds push on the surface
waters, forcing the surface waters to move in those same directions. Hence, surface currents generally flow in the
direction of the prevailing winds. The
prevailing winds blow from east to west near the equator, from west to east
near the midlatitudes, and from east to west again
near the poles. We will discuss why this
is the case later in the course. These
prevailing winds push the surface waters in these same directions. Thus, surface currents generally flow from
east to west near the equator, from west to east near the midlatitudes,
and from east to west again near the poles.
Consider a surface current
near the equator flowing from east to west, and suppose this surface current
runs into the east coast of a continent.
The flowing water will not spread out equally in both directions along
the east coast of the continent because of the Coriolis force. We will discuss the Coriolis force in detail
later in the course. For now, the
Coriolis force is caused by the rotation of planet
Earth and causes deflections to the right in the northern hemisphere and
deflections to the left in the southern hemisphere. We will discuss why this is the case later in
the course. The Coriolis force pushes on
the prevailing winds causing them to deflect, and these prevailing winds push
on the surface waters deflecting them as well.
In addition, the Coriolis force pushes directly on the surface currents,
thus also directly contributing to their deflection. Hence, when a surface current near the
equator runs into the east coast of a continent, the flowing water north of the
equator is deflected to the right (which is in fact northward), while the flowing
water south of the equator is deflected to the left (which is in fact
southward). The result is surface
currents flowing from the equator toward the poles along the east coast of the
continent. When these surface currents
reach the midlatitudes, the prevailing winds will
push the flowing water back into the ocean from west to east. If these surface currents run into the west
coast of another continent, the Coriolis force will deflect the midlatitude current in the northern hemisphere to the right
(which is in fact southward) and will deflect the midlatitude
current in the southern hemisphere to the left (which is in fact
northward). The result is surface
currents flowing from the poles toward the equator along the west coast of this
continent. The flowing water may then
arrive at the equator, where prevailing winds will push that water from east to
west again. We conclude that there are
circulations of surface currents in the ocean.
These are called oceanic gyres. The Greek root gyr-
means circle or wheel in words such as gyrate and gyroscope for example. Actually, the oceanic gyres we have discussed
are the subtropical oceanic gyres, since they are near the equator. There are subpolar oceanic gyres near the
poles. Subtropical oceanic gyres north
of the equator circulate clockwise, while subtropical oceanic gyres south of
the equator circulate counterclockwise.
Subpolar oceanic gyres near the North Pole circulate counterclockwise,
while subpolar oceanic gyres near the South Pole circulate clockwise. Although we will discuss subtropical oceanic
gyres in detail, we will only discuss subpolar oceanic gyres briefly, for
reasons that we will make clear shortly.
The surface current that
flows along the east coast of a continent from the equator toward the poles is
called the western boundary current of the subtropical oceanic gyre. The surface current that flows along the west
coast of a continent from the poles toward the equator is called the eastern
boundary current of the subtropical oceanic gyre. This nomenclature is some cause of
confusion. Nevertheless, every continent
ends where it meets an ocean, and every ocean ends where it meets a continent. Therefore, the western margin of every ocean
is the east coast of some continent, and the eastern margin of every ocean is
the west coast of some continent. If
City A is to the north of City B, then City B is to the south of City A! If one student sits to another student’s left, then the second student must be sitting to
the first student’s right! The western
boundary current of a subtropical oceanic gyre flows from the equator toward
the poles; hence, western boundary currents deliver warm water to the east
coast of a continent. The eastern
boundary current of a subtropical oceanic gyre flows from the poles toward the
equator; hence, eastern boundary currents deliver cool water along the west
coast of a continent. These boundary
currents of subtropical oceanic gyres help to moderate temperatures on planet
Earth. Western boundary currents deliver
warm water to the polar latitudes, moderating the cold temperatures at these
latitudes. Eastern boundary currents
deliver cool water to the equatorial latitudes, moderating the warm
temperatures at these latitudes.
Although the equatorial latitudes are still hot by human standards and
the polar latitudes are still cold by human standards, the temperature
difference is nevertheless moderate as compared with other planets, where
differences between equatorial temperatures and polar temperatures are extreme. The abundance of water covering planet Earth
together with these boundary currents as well as the water vapor in the
atmosphere (which we will discuss later in the course) all together moderate
temperature differences on planet Earth.
Without all of these effects, temperature differences between daytime
and nighttime and temperature differences between the equator and the poles
would be too extreme for life to exist on planet Earth.
As
we will discuss later in the course, the Coriolis force is weak near the
equator, and the Coriolis force becomes stronger and stronger further from the
equator toward the poles. Thus, the
strong Coriolis force pulls much of the water off of midlatitude surface currents as compared with surface
currents near the equator. This leaves
very little water by the time midlatitude surface
currents reach eastern boundary currents; thus, the eastern boundary current of
a subtropical oceanic gyre is weak and slow and
shallow. The weak Coriolis force pulls
very little water off of the surface currents near the
equator as compared with midlatitude surface
currents. This leaves most of the water
by the time surface currents near the equator reach western boundary currents;
thus, the western boundary current of a subtropical oceanic gyre is strong and fast and deep.
Actually, the Coriolis force does succeed in deflecting some water all
around the subtropical gyre. If
deflections are to the right in the northern hemisphere where subtropical
oceanic gyres circulate clockwise, and if deflections are to the left in the
southern hemisphere where subtropical oceanic gyres circulate counterclockwise,
the result is inward deflections directed toward the center of subtropical
oceanic gyres in both hemispheres. Thus,
there will be a higher elevation of sea level as water accumulates toward the
center of the subtropical oceanic gyre.
We deduce that there are varying elevations of sea level in the ocean,
literally mountains and valleys of water in the ocean! As another example of these varying elevations
of sea level, surface currents near the equator that flow from east to west
cause a mountain of water on the western margin of the ocean as water
accumulates at the east coast of some continent. There will be a valley of water on the
eastern margin of the ocean at the west coast of another continent. Since water must flow downhill, there will be
a weak surface current flowing from west to east along the equator. This is called the
equatorial countercurrent. This surface
current flows from west to east, against the direction of the surface currents
near the equator that flow from east to west.
The Latin root counter- means against in words such as counterattack,
counterintuitive, and counterclockwise for example. Although the prevailing winds near the
equator blow from east to west, the prevailing winds at the equator itself are
virtually nonexistent, as we will discuss later in the course. Therefore, the equatorial countercurrent can
flow virtually unhindered against the direction of the surface currents near
the equator. The equatorial
countercurrent is warm since it flows along the equator. The equatorial countercurrent is also weak and slow and shallow.
Caution: this discussion of the equatorial countercurrent is only
correct under normal circumstances; there are abnormal circumstances that we
will discuss shortly.
The North Atlantic
Subtropical Gyre circulates clockwise in the North Atlantic Ocean. The western boundary current of this gyre is
the Gulf Stream, which is strong and fast and deep and
delivers warm water to the east coast of North America. The Gulf Stream is named
for the Gulf of America (the Gulf of Mexico).
The eastern boundary current of the North Atlantic Subtropical Gyre is
the Canary Current, which is weak and slow and shallow
and delivers cool water to the west coast of Africa. The Canary Current is named
for the Canary Islands off the coast of northwestern Africa (off the coast of
Morocco). We have had some knowledge of
the North Atlantic Subtropical Gyre for roughly five hundred years. More than five centuries ago when Christopher
Columbus was planning his famous voyage across the Atlantic Ocean, he noticed
that midlatitude ocean currents come in from the
Atlantic Ocean toward Europe while ocean currents near the equator flow out
away from Africa into the Atlantic Ocean.
Therefore, when Christopher Columbus set sail from Spain with his three
ships the Niña, the Pinta, and the Santa María,
he first let the Canary Current carry the ships south. Then, the surface currents near the equator
carried the ships west into the Atlantic Ocean.
Finally, the Gulf Stream carried the ships north until they landed
probably at one of the islands of the Bahamas, although historians are not certain
precisely where they landed. Almost
three centuries after Christopher Columbus made his famous voyage (more than
two centuries before today), Benjamin Franklin mapped
the North Atlantic Subtropical Gyre.
Benjamin Franklin was one of the greatest founding fathers of the United
States; he signed both the Declaration of Independence and the Constitution of
the United States for example. Benjamin
Franklin was also a brilliant scientist; he discovered lightning is electricity
and invented the lightning rod and bifocal glasses for example. Benjamin Franklin was also the first person
to propose that the eruption of Laki Fissure in
Iceland in June 1783 was responsible for the bitterly cold winter of late 1783
to early 1784, as we discussed earlier in the course. The United States is currently composed of
fifty states, but the United States was born with only thirteen states along
the Atlantic coast (the east coast) of North America. Before these thirteen states were thirteen
states, they were thirteen colonies of the British Empire. In other words, England was the mother
country of these thirteen American colonies.
Benjamin Franklin noticed that when sailing across the Atlantic Ocean
from the colonies to the mother country, the trip took a shorter time as
compared to sailing across the Atlantic Ocean from the mother country back to
the colonies, which took a longer time.
This is because when sailing from Europe toward North America, we travel
from east to west against the midlatitude surface
current, which slows us down. When
sailing from North America toward Europe, we travel from west to east along
with the midlatitude surface current, which speeds us
up. Benjamin Franklin actually proposed
establishing a highway system in the North Atlantic Ocean. Rather like always remaining on the right
side of a street or a highway while riding horses (or driving cars today),
Benjamin Franklin suggested that ships would only sail directly across the
Atlantic Ocean when traveling from the colonies to the mother country (from
west to east). When sailing from the
mother country back to the colonies, Benjamin Franklin suggested that ships
should first let the Canary Current carry them south, then let surface currents
near the equator carry them west into the Atlantic Ocean, and then let the Gulf
Stream carry them north to arrive at the colonies. Although such a proposed trip from the mother
country to the colonies would be a much longer distance, it would take less
time since surface currents would carry ships faster as compared with sailing
directly across the Atlantic Ocean against the midlatitude
surface current. Today, we also
understand that the North Atlantic Subtropical Gyre brings warm water from the
equator not only along the east coast of North America but across the Atlantic
Ocean to Europe. This explains why
Europe is not as cold as Canada even though they are equally close to the North
Pole.
The South Atlantic
Subtropical Gyre circulates counterclockwise in the South Atlantic Ocean. The western boundary current of this gyre is the
Brazil Current, which is strong and fast and deep and
delivers warm water to the east coast of South America. The Brazil Current is named
for the country Brazil in northeastern South America. The eastern boundary current of the South
Atlantic Subtropical Gyre is the Benguela Current,
which is weak and slow and shallow and delivers cool
water to the west coast of Africa. The Benguela Current is named for the
city Benguela in the country Angola in southwestern
Africa. The southern boundary current of
the South Atlantic Subtropical Gyre is also important, as we will discuss.
Flowing from west to east
along the equator in the Atlantic Ocean is the Atlantic Equatorial
Countercurrent. This current is weak and slow and shallow and takes warm water from the east
coast of South America and delivers it to the west coast of Africa. Caution: this is only correct under normal
circumstances; there are abnormal circumstances that we will discuss shortly.
The North Pacific Subtropical
Gyre circulates clockwise in the North Pacific Ocean. The western boundary current of this gyre is
the Japan Current or the Kuroshio Current, which is strong
and fast and deep and delivers warm water to far-east Asia. This current is named for both the country
Japan in far-east Asia and for the dark color of the
water. The eastern boundary current of
the North Pacific Subtropical Gyre is the California Current, which is weak and slow and shallow and delivers cool water to the
west coast of North America. The
California Current is named for the State of
California on the west coast of North America.
The South Pacific Subtropical
Gyre circulates counterclockwise in the South Pacific Ocean. The western boundary current of this gyre is
the East Australia Current, which is strong and fast
and deep and delivers warm water to the east coast of Australia. The eastern boundary current of the South
Pacific Subtropical Gyre is the Peru Current or the Humboldt Current, which is weak and slow and shallow and delivers cool water to the
west coast of South America. This
current is named for both the country Peru in
northwestern South America and for the Prussian explorer Alexander von
Humboldt. The southern boundary current
of the South Pacific Subtropical Gyre is also important, as we will discuss.
Flowing from west to east
along the equator in the Pacific Ocean is the Pacific Equatorial
Countercurrent. This current is weak and slow and shallow and takes warm water from
Indonesia and delivers it to the west coast of South America. Caution: this is only correct under normal
circumstances; there are abnormal circumstances that we will discuss shortly.
The Indian Subtropical Gyre
circulates counterclockwise in the Indian Ocean. The western boundary current of this gyre is strong and fast and deep and delivers warm water to the east
coast of Africa. The eastern boundary
current of this gyre is the West Australia Current, which is weak
and slow and shallow and delivers cool water to the west coast of
Australia. The southern boundary current
of the Indian Subtropical Gyre is also important, as we will discuss.
Flowing from west to east
along the equator in the Indian Ocean is the Indian Equatorial
Countercurrent. This current is weak and slow and shallow and takes warm water from the east
coast of Africa and delivers it to Indonesia.
Caution: this is only correct under normal circumstances; there are
abnormal circumstances that we will discuss shortly.
There is a misconception
among North Americans and South Americans that supposedly
the Atlantic Ocean is warm and the Pacific Ocean is cold. This misconception is forgivable. Whenever North Americans or South Americans
go swimming in the Atlantic Ocean, they are swimming in the Gulf Stream or the
Brazil Current, as the case may be.
These are western boundary currents that bring
warm water along the east coast of these continents. Whenever North Americans or South Americans
go swimming in the Pacific Ocean, they are swimming in the California Current
or the Peru Current (the Humboldt Current), as the case may be. These are eastern boundary
currents that bring cool water along the west coast of these
continents. North Americans and South
Americans on the Atlantic coast (the east coast) are accustomed to swimming in
pleasantly-warm ocean temperatures due to western boundary currents; they are
unpleasantly surprised when they visit the Pacific coast (the west coast) and
try to go swimming in the unpleasantly-cold ocean temperatures due to eastern
boundary currents. Note that far-east
Asians and Australians have the opposite misconception: they believe that the
Pacific Ocean is warm! Again, this
misconception is forgivable. Whenever
far-east Asians or Australians go swimming in the Pacific Ocean, they are
swimming in the Japan Current (the Kuroshio Current) or the East Australia
Current, as the case may be. These are western boundary currents that bring warm water along
the east coast of these continents.
Africans have a similar misconception: they believe that the Atlantic
Ocean is cold! This is because whenever
Africans go swimming in the Atlantic Ocean, they are swimming in the Canary
Current or the Benguela Current, as the case may
be. These are eastern
boundary currents that bring cool water along the west coast of
Africa. Here is another similar
misconception: Africans believe that the Indian Ocean is warm, while
Australians believe that the Indian Ocean is cold. Whenever Africans go swimming in the Indian
Ocean, they are swimming in a western boundary current that brings warm water
along the east coast of Africa. Whenever
Australians go swimming in the Indian Ocean, they are swimming in an eastern
boundary current that brings cool water along the west coast of Australia. In summary, different peoples living on
different continents have different misconceptions about the temperature of
various oceans. In actuality, they are
only experiencing the temperatures of the particular surface currents that flow
along their continental coastlines.
There are presently only five
large gyres in the entire ocean: the North Atlantic Subtropical Gyre, the South
Atlantic Subtropical Gyre, the North Pacific Subtropical Gyre, the South
Pacific Subtropical Gyre, and the Indian Subtropical Gyre. There are several small subtropical gyres,
such as the Arabian Gyre and the Bengal Gyre.
There are also several small subpolar gyres, such as the Alaska Gyre and
the Bering Gyre (both near the North Pole) and the
Ross Gyre and the Weddell Gyre (both near the South Pole). There happens to be a continent at the South
Pole: Antarctica. Therefore, there is
insufficient room to establish a large subpolar gyre near the South Pole. The continents North America, Europe, and
Asia as well as the microcontinent Greenland leave insufficient room to
establish a large subpolar gyre near the North Pole. Thus, there are presently five large
subtropical gyres but no large subpolar gyres.
Again, there are several small subtropical gyres and several small
subpolar gyres. Due to the motion of the
tectonic plates, the shapes of continents and oceans change over millions of
years. Perhaps millions of years ago or
perhaps millions of years from now, all of the continents were or will be at
the equator, thus leaving insufficient room to establish large subtropical
gyres. There would only be large
subpolar gyres during such eras of Earth’s history. Perhaps there were or perhaps there will be
other eras of Earth’s history when the continents do leave sufficient room to
establish a couple large subtropical gyres as well as a couple large subpolar
gyres. We happen to be alive during an
era of Earth’s history when there are five large subtropical gyres and no large
subpolar gyres, but again there are several small subtropical gyres and several
small subpolar gyres. The most
simplistic definition of an ocean is an enormous body of water, but a more
advanced definition of an ocean is an enormous body of water with a large gyre
circulating within it. By this more
advanced definition, there are presently five oceans: the North Atlantic Ocean,
the South Atlantic Ocean, the North Pacific Ocean, the South Pacific Ocean, and
the Indian Ocean. Note that this more
advanced definition of an ocean forces us to count the Atlantic Ocean twice and
to count the Pacific Ocean twice. Also notice that we are not counting the so-called Arctic
Ocean at all. Indeed, the so-called
Arctic Ocean is very small and very shallow as compared with the other
oceans. For all of these reasons, the
so-called Arctic Ocean should not be regarded as an ocean and should be renamed
the Arctic Sea or the North Pole Sea.
During other eras of Earth’s history, there were (and there will be)
different numbers of oceans with completely different shapes from the five
present-day oceans.
All oceanic gyres are caused by a combination of the prevailing winds and the
Coriolis force pushing on the surface waters as well as the configuration of
the continents interrupting the flow of surface currents. There is only one latitude where we could actually
sail all the way around the world without any continents interrupting our
journey: the midlatitudes of the southern
hemisphere. Without any continents to
interrupt their flow, the southern boundary current of the South Pacific
Subtropical Gyre and the southern boundary current of the South Atlantic
Subtropical Gyre and the southern boundary current of the Indian Subtropical
Gyre merge to become the strongest current in the entire world: the Antarctic
Circumpolar Current. This current
surrounds the continent Antarctica, which is at the South Pole. The Latin root circum-
means around in words such as circumference, circumscribe, and circumvent for
example; hence, the word circumpolar literally means around the pole. The Antarctic Circumpolar Current is the
strongest current in the entire world and is responsible for the rough waters
at the midlatitudes of the southern hemisphere. These rough waters are
commonly known as the Roaring Forties, the Furious Fifties, and the
Screaming Sixties. The words forties,
fifties, and sixties refer to degrees south latitude. These rough waters were
first encountered by Ferdinand Magellan who led the first crew to
circumnavigate the entire world.
As Magellan and his crew sailed southward into the South Atlantic Ocean,
they encountered these rough waters.
After they sailed around the southern tip of South America through the
strait that would later be named for Magellan himself, they sailed northward
out of these rough waters and into more calm waters. Hence, Magellan called this new ocean the
Pacific Ocean, which literally means peaceful ocean. The word pacific is derived
from the Latin word pax, meaning peace. Other words derived from the Latin word pax include pacify, pacifist, and pacifism. In actuality, the entire Pacific Ocean is not
peaceful. Magellan and his crew happened
to be sailing into the South Atlantic Ocean while entering the rough waters of
the midlatitudes of the southern hemisphere, and they
happened to be sailing into the South Pacific Ocean while leaving the rough
waters of the midlatitudes of the southern
hemisphere. These
rough waters were also encountered by Francis Drake, the first Englishman to
circumnavigate the entire world.
As Drake and his crew sailed southward into the South Atlantic Ocean,
they encountered these rough waters.
After they sailed around the southern tip of South America through the
passage that would later be named for Drake himself,
they sailed northward out of these rough waters into more calm waters. If the first person to ever circumnavigate
the world happened to be from far-east Asia instead of
from Europe, he would have first encountered the Roaring Forties, the Furious
Fifties, and the Screaming Sixties while sailing southward into the South
Pacific Ocean. Perhaps after sailing
around the southern tip of South America through the strait or through the
passage that perhaps would later be named for this
hypothetical explorer, his hypothetical crew would have then sailed northward
out of these rough waters and into more calm waters in the South Atlantic
Ocean. If this had happened, the
Atlantic Ocean would have been named the peaceful
ocean instead.
The Antarctic Circumpolar
Current surrounds Antarctica and therefore isolates Antarctica from the rest of
the world. As a result, Antarctica and
the surrounding waters are very cold. In
addition, there is a continental effect at the South Pole; after all,
Antarctica is a continent! There is a
marine effect at the North Pole, since the so-called Arctic Ocean is a body of
water. For all of these reasons,
temperatures at the North Pole are mildly cold as compared with temperatures at
the South Pole, which are more severely cold.
Hundreds of millions of years ago, other continents were
connected to Antarctica forming a supercontinent. Beginning roughly two hundred million years
ago, the other continents began to rip off of
Antarctica. The final continent to rip off of Antarctica was South America, which explains why
South America is the closest continent to Antarctica today. The Drake Passage between Antarctica and
South America is only roughly one thousand kilometers wide. When South America ripped off
of Antarctica roughly thirty million years ago, Antarctica became
isolated at the South Pole. With no
further continental obstructions at the midlatitudes
of the southern hemisphere, the Antarctic Circumpolar Current was established,
further isolating Antarctica. Hence,
Antarctica became extremely cold, and our entire planet Earth plunged into what
is called the Current Ice Age. The Current Ice Age began roughly thirty
million years ago and continues to the present day. The Current Ice Age will continue for many
more millions of years as long as Antarctica is isolated at the South Pole
surrounded by the Antarctic Circumpolar Current that further isolates
Antarctica. There are
only two scenarios that can end the Current Ice Age. In one scenario, Antarctica may move off of the South Pole, which would make it less cold. This would also interrupt the Antarctic
Circumpolar Current, which would contribute to warming temperatures. Alternatively, another continent may move to
the South Pole and collide with Antarctica.
This would end the isolation of Antarctica, making it less cold. This would also interrupt the Antarctic
Circumpolar Current, again contributing to warming temperatures. Whether Antarctica moves away from the South
Pole or another continent moves toward the South Pole, it takes millions of
years for tectonic plates to move significantly, as we discussed earlier in the
course. Therefore, the Current Ice Age
will continue for millions of more years to come. During other eras of Earth’s history millions of years ago and perhaps millions of years
from now, the configuration of the continents and the oceans was or will be
such that other continents were or will be isolated at one or both poles
surrounded by surface currents that further isolated those continents. In other words, there were and perhaps there
will be other ice ages in Earth’s history.
We will discuss these other ice ages in Earth’s history later in the
course.
Asia is the largest continent
in the world; therefore, Asia experiences a strong continental effect. In other words, Asia becomes extremely hot in
the summertime and extremely cold in the wintertime. If Asia is very hot in the summertime, the
air above Asia is hot as compared with the air above the Indian Ocean. As we will discuss later in the course, hot
air is at a low pressure. So, the hot air above Asia is at a lower pressure as
compared with the air above the Indian Ocean which is at a comparatively higher
pressure. As we will also discuss later
in the course, wind blows from high pressure toward low pressure. Therefore, summertime winds tend to blow from
the Indian Ocean (higher air pressure) toward Asia (lower air pressure). These winds are humid, since water evaporates
from the Indian Ocean to the overlying air.
Therefore, summertime humid winds blow from the Indian Ocean toward
Asia, and these humid winds bring heavy rains and even floods to India. This is called the
southwest monsoon, since the humid winds are blowing from the southwest (toward
the northeast). The reverse occurs in
the wintertime. Since Asia is very cold
in the wintertime, the air above Asia is cold as compared with the air above
the Indian Ocean. As we will discuss
later in the course, cold air is at a high pressure. So, the cold air
above Asia is at a higher pressure as compared with the air above the Indian
Ocean, which is at a comparatively lower pressure. Again, wind blows from high pressure toward
low pressure. Therefore, wintertime
winds tend to blow from Asia (higher air pressure) toward the Indian Ocean
(lower air pressure). These winds are
dry, since there is much less water on a continent to evaporate to the
overlying air. Therefore, wintertime dry
winds blow from Asia toward the Indian Ocean, and these dry winds bring
droughts to India. This is called the northeast monsoon, since the dry winds are
blowing from the northeast (toward the southwest). It is a common misconception that the word
monsoon means stormy weather such as rain, but this is not correct. The wet season and the dry season are both
monsoons. The word monsoon is derived from an Arabic word that simply means season or
time of year. At the midlatitudes,
the seasons are generally regarded as summertime and
wintertime. In India, the seasons are instead regarded as these two monsoons: the southwest
(wet) monsoon and the northeast (dry) monsoon.
The southwest (wet) monsoon is analogous to midlatitude
summertime, and the northeast (dry) monsoon is analogous to midlatitude
wintertime.
Something analogous to the
monsoons in the Indian Ocean also occurs in the Pacific Ocean. Under normal circumstances, the prevailing
winds push the surface currents near the equator from east to west. This brings humid winds and moderate rain to
Indonesia, leaving moderately dry conditions in Ecuador and Peru. Eventually so much water accumulates in Indonesia
that all of this water begins to flow back to the Pacific Ocean, from west to
east. The water may flow so strongly
that the Pacific Equatorial Countercurrent intensifies and pushes the other
surface currents in this west-to-east direction as well. The result is rain in Ecuador and Peru and
dry conditions in Indonesia. People
living in Ecuador and Peru have noticed for centuries that on occasion ocean
currents come in from the Pacific Ocean bringing rains and even floods. This always seemed to occur around
Christmastime. Therefore, people living
in Ecuador and Peru named this El Niño, which is Spanish for the child as in
the Christ Child, because Christmas is the celebration of the birth of Jesus
Christ. These people believed that El
Niño was a small, local occurrence.
Today we realize that El Niño is actually occurring across the entire
Pacific Ocean. Since the Pacific Ocean
is so enormous, El Niño is actually a global phenomenon. So much water may build up in Ecuador and
Peru during an El Niño that the water is forced to
flow back to the Pacific Ocean, from east to west. This is the direction of surface currents
near the equator under normal circumstances, but these conditions are beyond
normal or extreme normal. The surface
currents near the equator in the Pacific Ocean become so strong from east to
west that the Pacific Equatorial Countercurrent may vanish or perhaps even flow
in this east-to-west direction as well.
Under normal circumstances, there is moderate rain in Indonesia and
moderately dry conditions in Ecuador and Peru.
During these extreme-normal circumstances, there are extreme floods in
Indonesia and extreme droughts in Ecuador and Peru. Since these extreme-normal circumstances are
the opposite of El Niño, oceanologists/oceanographers
have named it La Niña, since El Niño is actually Spanish for the little boy and
La Niña is Spanish for the little girl.
La Niña is just as much a global phenomenon as El Niño. Either an El Niño or a La Niña may last as
short as a few months, and either an El Niño or a La Niña can last as long as a
couple years. Ironically, normal
circumstances are actually the most rare
circumstances; normal conditions only occur briefly while switching from an El
Niño to a La Niña or the other way around.
For reasons we do not
understand, there are decades of El Niño domination followed by decades of La
Niña domination and then back again.
This alternation between El Niño domination and La Niña domination is called the Pacific Decadal Oscillation (PDO). During decades
of El Niño domination, although there are still La Niñas,
there are more El Niños and they are stronger and
they last longer. During decades of La
Niña domination, although there are still El Niños,
there are more La Niñas and they are stronger and
they last longer. Again, we do not
understand how or why the Pacific Decadal Oscillation (PDO)
occurs. Several years ago, we began a
period of La Niña domination preceded by a period of El Niño domination. This may explain why hurricane seasons were
so horrific during the last decade of the previous century (the twentieth century)
and the first decade of the current century (the twenty-first century), since
those decades were during a period of El Niño domination. This may also explain why hurricane seasons
have been relatively mild during the last several years, since we are currently
in a period of La Niña domination.
Caution: we may still have powerful hurricanes during decades of La Niña
domination, just as we may still have mild hurricanes during decades of El Niño
domination. Another oscillation similar
to the Pacific Decadal Oscillation (PDO) occurs in
the Atlantic Ocean; this is called the Atlantic Multidecadal Oscillation (AMO). Direct observations of the Pacific Decadal
Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO)
only stretch back several decades.
However, measurements of the radioactive isotope carbon-fourteen 
within trees have revealed that the Pacific
Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO) each
undergo on average roughly sixty-year cycles, where one complete Pacific
Decadal Oscillation (PDO) for example consists of
roughly three decades of El Niño domination followed by roughly three decades
of La Niña domination. The Pacific
Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO)
together strongly influence the global climate of our planet Earth, as we will
discuss later in the course.
Eastern boundary currents
bring cool water from the poles to the equator along the west coast of
continents. This cool water destroys the
thermocline, which destroys the pycnocline, thus
causing the mixing of surface water with deep water at the equator. This results in an abundance of nutrients in
these particular equatorial waters.
There is certainly abundant sunlight at the equator. Abundant sunlight and abundant nutrients
together result in high biological productivity in these particular equatorial
waters. This is an
exception to the marine deserts at the equatorial waters that we discussed. For example, there is a thriving caviar industry
off the coast of Ecuador and Peru.
Caution: this analysis is only correct under normal circumstances. Even this
Ecuadorian-Peruvian caviar industry is disrupted by occasional El Niños.
Deep currents are vertical
flows of water within the deep ocean caused by density variations of ocean
waters. Low density
water will rise and flow upward, while high density water will sink and flow
downward. The poles are cold throughout
the entire year. As we discussed, the
South Pole is colder than the North Pole due to the continental effect together
with the Antarctic Circumpolar Current surrounding and isolating
Antarctica. Hence, the waters around
Antarctica are so cold that they sink all the way to the bottom of the ocean
and flow along the ocean floor. This is called Antarctic Bottom Water, and it is the coldest
liquid water in the entire world. The
water at the so-called Arctic Ocean at the North Pole is also cold, and so it
sinks almost to the ocean floor and flows on top of the Antarctic Bottom Water. This is called
Arctic Deep Water, and it is the second coldest liquid water in the entire
world. These waters flow along the ocean
floor for perhaps hundreds of years or perhaps thousands of years; we are not
certain. These waters may eventually
warm through perhaps submarine igneous activity. These warmer waters rise to the surface of
the ocean where the prevailing winds and the Coriolis force push them, turning
them into surface currents. Eventually,
these surface waters may end up at either the North Pole or the South Pole
where they will become sufficiently cold to sink again, completing the entire
cycle. Again, we are not certain how
long it takes ocean water to complete this entire cycle: perhaps centuries or
perhaps millennia. Not much more is known about deep currents.
Physical Oceanology/Oceanography: Ocean Waves
Ocean currents are actually
flowing bodies of water in the ocean, rather like
rivers are flowing bodies of water on the continents. However, ocean waves are much more abstract. A wave is a propagating (traveling)
disturbance. This means that when a
water wave propagates (travels), the water does not necessarily actually move
in the direction that the wave is propagating (traveling). The water could be moving in the direction of
the wave, but not necessarily. The
following example will make this more clear. Imagine water flowing in a river downstream,
and suppose we throw a rock or pebble into the river. When the rock or pebble lands in the river
water, the impact causes waves that propagate (travel) outward in all
directions from where the rock or pebble plunged into the water. These waves propagate (travel) in all
directions. Therefore, although some of
these waves propagate downstream in the direction of the flowing water, some of
these waves actually propagate (travel) upstream in the opposite direction of
the flowing water! This reveals how
abstract waves are. Ocean currents are
clear: they are flowing bodies of water in the ocean. Ocean waves are abstract: they are propagating
(traveling) disturbances within the ocean.
Let us discuss waves in general while always keeping in mind that we
will eventually apply everything we have discussed about waves in general to
ocean waves in particular.
A wave is a propagating (traveling)
disturbance. This implies that a wave
requires a medium through which to propagate, since we cannot have a
propagating disturbance if there is nothing to disturb! A transverse wave is a wave where the
direction of the disturbance is perpendicular to the direction of propagation,
while a longitudinal wave is a wave where the direction of the disturbance is
parallel and antiparallel to the direction of propagation. Light is a real-life example of a transverse
wave, while sound is a real-life example of a longitudinal wave. A wave can have a component of its
disturbance perpendicular to the direction of propagation and another component
parallel and antiparallel to the direction of propagation. In other words, a wave can be both transverse
and longitudinal. Water waves in the
ocean are a real-life example of a wave that is both transverse and
longitudinal. The maximum magnitude of a
wave’s disturbance is called the wave amplitude, and
these amplitudes occur at what are called crests (maximum positive disturbance)
and troughs (maximum negative disturbance).
The wave height is the amount of disturbance between a crest and a
trough. In other words, the wave height
is double the wave amplitude (or the wave amplitude is half of the wave
height). The distance from one crest to
the next crest (which is also the distance from one trough to the next trough) is called the wavelength of the wave and is always given the
symbol λ (the lowercase Greek letter lambda). Caution: the word wavelength is misleading,
since it may lead us to conclude that the wavelength is the length of the
entire wave, which it is not. The
wavelength of a wave is the length of only one cycle of the wave. The frequency of a wave is the number of
crests passing a point every second as well as the number of troughs passing a
point every second. We may also
interpret the frequency of a wave as how many cycles or
oscillations or vibrations the wave executes every second. In other words, the frequency of a wave is
how frequently the wave is vibrating
or oscillating, which is why it is called the frequency! A high-frequency wave is oscillating
(vibrating) many cycles every second, while a low-frequency wave is oscillating
(vibrating) a small number of cycles every second. We will use the symbol f for frequency, and its units are cycles per second or vibrations
per second or oscillations per second.
This unit is called a hertz with the
abbreviation Hz, named for the German physicist Heinrich Hertz. Again, one hertz (Hz) is one cycle per second
or one vibration per second or one oscillation per second. A kilohertz is one thousand hertz or one
thousand cycles per second, since the metric prefix kilo- always means
thousand. For example, one kilometer is
one thousand meters, and one kilogram is one thousand grams. A megahertz is one million hertz or one
million cycles per second, since the metric prefix mega- always means
million. On the amplitude-modulation radio
band (AM radio), the radio-station numbers are kilohertz, while on the frequency-modulation
radio band (FM radio), the radio-station numbers are megahertz.
The speed of a wave is a
function of the properties of the medium through which the wave
propagates. For example, the speed of
sound is some speed through gases such as air, a faster speed through liquids,
and an even faster speed through solids.
The speed of sound through air is not even fixed; the speed of sound
through air actually changes as the temperature of the air changes. As another example, the speed of light is
some speed through gases such as air, a slower speed through liquids such as
water, and an even slower speed through solids such as glass. The speed of any wave with wavelength λ
and frequency f is determined by the
equation v = f λ, where v is the
speed (the velocity) of propagation of the wave. If we solve this equation for the frequency,
we deduce that f = v / λ. Therefore, frequency and wavelength are
inversely proportional to each other.
Waves with higher frequencies have shorter wavelengths, while waves with
lower frequencies have longer wavelengths.
In addition to the properties of the medium of propagation, the speed of
a wave is almost always also a function of the wave’s
own wavelength or frequency thus resulting in dispersion, as we will discuss
shortly.
The amplitude of any wave
determines its energy. In particular,
the energy of a wave is directly proportional to the square of its
amplitude. Therefore, a wave with a
larger amplitude has more energy, while a wave with a smaller amplitude has
less energy. For example, the amplitude
of a sound wave determines its loudness.
A sound wave with a larger amplitude is more loud,
while a sound wave with a smaller amplitude is less loud (more quiet). As another example, the amplitude of a light
wave determines its brightness. A light
wave with a larger amplitude is more bright, while a
light wave with a smaller amplitude is less bright (more dim). The frequency of a wave is difficult to
interpret physically, and so we will interpret the frequency of a wave on a
case-by-case basis. For example, the
frequency of a sound wave is its pitch, meaning that a sound wave with a higher
frequency has a higher pitch while a sound wave with a lower frequency has a
lower pitch. As another example, the
frequency of a visible light wave is its color.
In particular, a visible light wave with a high frequency is blue or
violet, a visible light wave with a low frequency is red or orange, and a
visible light wave with an intermediate frequency is yellow or green. In order starting from the lowest frequency
(which is also the longest wavelength), the colors of visible light are red,
orange, yellow, green, blue, indigo, and violet at the highest frequency (which
is also the shortest wavelength). This
is why the colors of the rainbow are in this order; a rainbow reveals the
correct sequence of colors as determined by either the frequency or the
wavelength. We can memorize this
sequence of colors with a mnemonic.
Using the letters r for red, o for orange, y for yellow, g for green, b
for blue, i for indigo, and v for violet, we
construct a fanciful name of an imaginary person: Roy G. Biv.
Whereas the frequency and the
wavelength of a wave are constrained to one another through the equation v = f
λ, no universal equation constrains the amplitude with the frequency. Therefore, a wave can have a large amplitude
and a high frequency, a wave can have a large amplitude and a low frequency, a
wave can have a small amplitude and a high frequency, and a wave can have a
small amplitude and a low frequency. In
other words, all of these combinations are physically possible. For example, a sound wave
with a large amplitude and a high frequency is a loud high-pitch sound, a sound
wave with a large amplitude and a low frequency is a loud low-pitch sound, a
sound wave with a small amplitude and a high frequency is a quiet high-pitch
sound, and a sound wave with a small amplitude and a low frequency is a quiet
low-pitch sound. As another example, a visible light wave with a large amplitude and
a high frequency is bright blue, a visible light wave with a large amplitude
and a low frequency is bright red, a visible light wave with a small amplitude
and a high frequency is dim blue, and a visible light wave with a small
amplitude and a low frequency is dim red.
Consider any wave propagating
in a certain medium that encounters a second medium. This wave is called
the incident wave. At the boundary
between the two media, a part of the wave will bounce back into the first
medium while the rest of the wave will be transmitted
into the second medium. The wave that
bounces back into the first medium is called the
reflected wave, while the wave that is transmitted into the second medium is
called the refracted wave. We will make
clear the meanings of the words reflection and refraction shortly. Any line perpendicular to the boundary
between the two media is called the normal to the
boundary, since the word normal in physics and engineering means
perpendicular. The angle between the
incident wave and the normal is called the angle of
incidence with the symbol θ1. The angle between the reflected wave and the
normal is called the angle of reflection with the
symbol θ3. The angle between the refracted wave and the
normal is called the angle of refraction with the
symbol θ2. The Law of Reflection states that θ1 = θ3
in all cases. In other words, the angle
of incidence is equal to the angle of reflection in all cases for all
waves. Reflection is the bouncing of a
part of a wave off of another medium with no change in
angle with respect to the normal. The
Law of Refraction states sin(θ1)/v1 =
sin(θ2)/v2, where v1 is
the speed of the wave in the first medium and v2 is the speed of the
refracted wave in the second medium.
Refraction is the bending of a wave due to a change in speed of the
wave. According to the Law of
Refraction, the angle of refraction θ2
is smaller than the angle of incidence θ1
if the transmitted speed v2
is slower than the incident speed v1, while the angle of refraction θ2 is larger than the angle of incidence θ1 if the transmitted speed v2 is
faster than the incident speed v1. More
plainly, a wave is refracted (bent) toward
the normal if the transmitted wave propagates slower than the incident wave,
while a wave is refracted (bent) away
from the normal if the transmitted wave propagates faster than the incident
wave.
When two or more waves of the
same nature (the same type) occupy the same space at the same time, they add
together to become a combined wave. This
is called interference. Caution: sometimes addition is indeed an
addition, but sometimes addition is actually a subtraction. For example, positive five added with
positive three yields positive eight, and negative five added with negative
three yields negative eight. However,
positive five added with negative three yields positive two. Notice that two is actually the difference
between five and three. Again, whereas
sometimes addition is indeed an addition, sometimes addition is actually a
subtraction. When two waves of the same
nature (the same type) occupy the same space at the same time, crest and crest
may meet, trough and trough may meet, or crest and trough may meet. The crests of a wave are locations of maximum
positive disturbance, and the troughs of a wave are locations of maximum
negative disturbance. When crest and
crest meet, the waves interfere with one another to become a combined wave with
an even greater positive disturbance, since adding positive numbers together
yields even greater positive numbers.
When trough and trough meet, the waves interfere with one another to
become a combined wave with an even greater negative disturbance, since adding
negative numbers together yields even greater negative numbers (in
magnitude). Either of these scenarios is called constructive interference, since the combined wave
has a larger amplitude than the individual waves that interfered to form the
combined wave. When crest and trough
meet, the waves interfere with one another to become a combined wave with a
smaller disturbance, since adding a positive number with a negative number
actually results in subtraction, such as adding positive five with negative
three to yield positive two. This
scenario is called destructive interference, since the
combined wave has a smaller amplitude than some of the individual waves that
interfered to form the combined wave.
Any type of interference between these two extremes is
called mixed interference.
Consider many waves with many
different wavelengths (or frequencies) that interfere with one another to form
a combined wave. The speed of the
individual waves is called the phase speed, while the
speed of the combined wave is called the group speed. Suppose all the
individual waves move at the same speed.
So, these individual waves will all move together. The resulting combined wave will then move at
the same speed as the individual waves.
In other words, the group speed and the phase are equal to each
other. Now suppose instead that the
individual waves all move at different speeds.
The combined wave will then spread as faster-moving waves pull out ahead
of slower-moving waves that lag behind.
This is called dispersion. The verb to disperse means to spread
out. The combined wave spreads because
the individual waves all move at different speeds. The combined wave will also move at a
different speed. In other words,
dispersion is the spreading of a wave because the phase speed and the group
speed are different from each other.
When the individual waves all move at the same speed, they will all move
together, resulting in the combined wave also moving at that same speed. The combined wave does not spread, since all the individual waves move together. This is called no
dispersion. A rainbow results from the
dispersion of light within water or glass.
White light is a combination of individual light waves with different
wavelengths or frequencies (different colors).
When white light propagates within water or glass, the different colors
all propagate at different speeds. This
also causes these colors to refract (to bend) at different angles
which separates the individual colors from each other, thus forming a
rainbow.
Diffraction is the bending of
a wave, but this is a different bending than the refraction of a wave. Refraction is the bending of a wave due to a
change in speed of the wave, but diffraction is the bending of a wave without
involving a change in speed of the wave.
Diffraction is the bending of a wave around obstacles. However, if the wavelength of a wave is small
compared with the size of the obstacles, this diffractive bending will be
negligible, and the wave will seem to propagate in straight lines. This is why light from a flashlight or even
more so from a laser pointer appears to travel a straight path. The wavelength of visible light waves is so
small compared with the sizes of everyday obstacles around us that the
diffraction of visible light is not noticeable.
However, the wavelength of sound waves is not small compared with the sizes
of everyday obstacles around us, such as hallways and doorways. Thus, the diffraction of sound is quite
severe. This is why we can hear someone
speaking who is nevertheless standing around a corner. The wavelength of sound is not small compared
to the size of the hallway; therefore, the sound diffracts around the corner
instead of traveling a simple straight path.
All water waves can be divided into three categories: deep-water waves,
shallow-water waves, and transitional waves.
Deep-water waves are water waves in such deep water that the seabed is
deeper than the bottom of the wave, called the wave base. Therefore, the phase speed of deep-water
waves is independent of the ocean depth; the phase speed of deep-water waves is
only a function of the wavelength of the wave.
For these deep-water waves, the group speed is half of the phase speed
(or the phase speed is double the group speed).
This can be demonstrated with the following
experiment. Suppose we throw a pebble
into a lake or pond. The plunging of the
pebble into the water causes deep-water waves that propagate outward in all
directions from where the pebble collided with the water. Although there are many
minor ripples propagating outward, there is often only one major ripple that
propagates outward. The major
ripple propagates twice as slow as the minor ripples (or the minor ripples
propagate twice as fast as the major ripple).
This reveals that the group speed is half of the phase speed (or the
phase speed is double the group speed) for deep-water waves. Shallow-water waves are waves in such shallow
water that the phase speed is only a function of the ocean depth; the phase
speed is independent of the wavelength of the wave. Transitional waves propagate where the ocean
is not too deep and not too shallow.
Thus, the phase speed is a function of both the wavelength of the wave
and the ocean depth. These transitional
waves are the most difficult water waves to analyze mathematically, since both
the ocean depth and the wavelength of the wave remain in the wave equation that
describes the wave. We will discuss why
these are called transitional waves shortly.
Ocean waves are formed when winds cause small vibrations of the surface
water. These are
commonly known as ripples, but they are more correctly called capillary
waves, since it is the surface tension or more correctly the capillarity that
restores the vibrations to a steady level.
Now suppose the wind speed picks up, and suppose the wind blows for a
longer duration of time. Also, suppose the wind blows for a further distance; the
distance that the wind blows is called the wind fetch. The wind adds more and more energy to the
water waves, making the amplitude larger and larger. Eventually, the amplitude is large enough
that gravity becomes the restoring force instead of the capillarity or surface
tension. The amplitude may continue to
grow as winds continue to add energy to the water waves. When the amplitude becomes too large, the
wave becomes unstable, and the wave breaks.
Caution: these are not the breakers we see at the
beach. We are discussing breakers
in the middle of the ocean, not breakers near the continent. In order to avoid confusion between these two
types of breakers, we will use two different terms for these two different
types of breakers. Breakers in the
middle of the ocean are called whitecaps. We will reveal the term for breakers near the
continent that we see at the beach shortly.
The formation of whitecaps (the breaking of ocean waves) does not
prevent winds from continuing to blow thus forming new ocean waves, which
themselves may soon break as whitecaps.
Eventually, an equilibrium is established
between forming waves and whitecaps.
These are called fully-developed sea
waves. These fully-developed
sea waves may leave the winds that created them in the first place and
propagate on their own. It is a common misconception that we must continuously push a wave
to keep it propagating, but this is false. We may need a force to create a wave in the
first place, but we do not need to continuously apply
that force to keep the wave propagating.
This can be demonstrated with waves on a long
spring. We may need to wiggle the spring
once to create a wave on the spring, but we do not need to
continuously wiggle the spring to keep the wave propagating; the wave
our first wiggle created propagates along the spring on its own without any
additional force pushing it. Any wave
that propagates on its own without any force pushing it is
called a free wave. Free waves in
the ocean are called swells. To summarize, first we have capillary waves
or ripples which then become fully-developed sea waves
which then become swells.
A rogue wave or monster wave is
an ocean wave with a giant crest (positive amplitude) that appears suddenly
with no warning and disappears just as suddenly as it appeared. The existence of these rogue waves or monster
waves has been a legend for thousands of years, but direct observations using
laser sensors and marine buoys starting in the year 1995 have proven their
existence. Rogue waves or monster waves
form completely accidentally. There are
thousands of waves in the ocean propagating in many different directions with
different speeds and different wavelengths.
As they pass through each other, these waves interfere with each
other. On most occasions, they suffer
from mixed interference, although sometimes they suffer from either
constructive interference or destructive interference. Suppose a large number of ocean waves happen
to pass through one another in such a way that all their crests happen to be at
the same place at the same time. We
would then have constructive interference from all of these waves, thus forming
a single combined wave with a giant crest (giant positive amplitude). This is a monster wave or rogue wave. The individual waves will pass through each
other, and so all of their crests will no longer be at
the same place at the same time. The
giant positive amplitude quickly decreases, and the rogue wave or monster wave
disappears just as suddenly as it appeared.
Many small ships are lost at sea without a trace every year. On occasion, a large ship is lost at sea
without a trace. These missing ships are probably swallowed by monster waves or rogue waves.
As a deep-water wave
approaches the continent, the ocean depth shallows. Eventually, the ocean depth is less deep but
not particularly shallow yet either. So, the deep-water wave becomes a transitional wave. As the wave continues to approach the
continent, the ocean depth becomes sufficiently shallow that the wave becomes a
shallow-water wave. We now understand
why transitional waves are called as such;
transitional waves are ocean waves that are transitioning from deep-water waves
to shallow-water waves. As the ocean
depth becomes more shallow, the ocean waves slow
down. This permits other ocean waves to
catch up to them. When the waves are at
the same place at the same time, interference occurs. Although the result is
usually mixed interference, either constructive interference or
destructive interference may also occur.
With constructive interference, the combined wave has a larger crest
(larger positive amplitude). If the
crest becomes too large, the wave becomes unstable, and the wave breaks. We see these near-continent breakers while at
the beach. Whereas breakers in the
middle of the ocean are called whitecaps, breakers
near the continent are called surf.
Humans have invented a sport where they ride surf. This sport is called
surfing, and persons who partake of this sport are called surfers. Caution: we must not confuse surfers with surfmen. Surfers are
persons who partake of the sport surfing, while surfmen
are persons who are trained to go out into the ocean
to rescue victims of shipwrecks.
There are several examples of
words that we may believe should exist in the English language, but nevertheless they do not exist in the English
language. For example, there is no word
readable in English; the correct word is legible. There is no word eatable in English; the
correct word is edible. There is no word
drinkable in English; the correct word is potable. There is no word explainable in English; the
correct word is explicable. There is no
word imitateable in English; the correct word is
imitable. Relevant to
oceanology/oceanography, there is no word shallowing in English; the correct
word is shoaling. The gerund (verbal
noun) shoaling is derived from the noun shoal, which is a part of the ocean
that is more shallow than the surrounding waters. For example, ships often run aground (become
stuck and hence stranded) in the shoals of the ocean near land. As waves approach the continent, the ocean
depth becomes more shallow. If the shoaling is gradual, the ocean waves
gradually slow down, and other ocean waves gradually catch up to them. Gradual interference results, with on
occasion gradual constructive interference, causing the waves to break gently. This is the weakest type of surf, called
spilling breakers. If the shoaling is
abrupt, the ocean waves suddenly slow down, and other ocean waves suddenly
catch up to them. Sudden interference
results, with on occasion sudden constructive interference, causing the waves
to break powerfully. This is the
strongest type of surf, called surging breakers. In between these two extremes are plunging
breakers caused by shoaling that is not particularly abrupt but also not
particularly gradual.
The surf on the Atlantic
coast (the east coast) of the United States is predominantly spilling breakers,
while the surf on the Pacific coast (the west coast) of the United States is
predominantly surging breakers. This is
for three reasons. Firstly, the east
coast of North America is a passive continental margin, making the shoaling
gradual. Ocean waves coming in from the
Atlantic Ocean first encounter the continental rise and then the continental
slope and then the continental shelf before reaching the shore. The west coast of North America is an active continental
margin, making the shoaling abrupt.
Secondly, the United States is at the midlatitudes,
and the midlatitude prevailing winds blow from west
to east. Therefore, these prevailing
winds blow with ocean waves that come in from the Pacific Ocean, making these
ocean waves stronger. However, these
prevailing winds blow against ocean waves that come in from the Atlantic Ocean,
making these waves weaker. Thirdly, even
if winds happened to be blowing from the east to the west in the Atlantic Ocean
to reinforce ocean waves propagating toward the east coast of North America,
the Atlantic Ocean is a much smaller ocean as compared with the Pacific
Ocean. Therefore, the wind fetch is
smaller, and hence ocean waves cannot be made
particularly strong. The Pacific Ocean
is a much larger ocean as compared with the Atlantic Ocean. Therefore, the wind fetch can be much
greater, and hence ocean waves can be made
particularly strong. All three of these
factors cooperate to make the surf predominantly surging breakers on the west
coast of the United States and predominantly spilling breakers on the east
coast of the United States. If we regard
any sport as more exciting when it is more dangerous, then we would regard
surfing as better on the west coast of the United States as compared with
surfing on the east coast of the United States, where it is lame. Surfers from the west coast
(the Pacific coast) of the United States who attempt to go surfing on the east
coast (the Atlantic coast) of the United States always feel that they are on a
kiddie ride, whereas surfers on the east coast (the Atlantic coast) of the
United States who attempt to go surfing on the west coast (the Pacific coast)
of the United States always get wiped out!
Tsunami are the second
longest wavelength waves in the entire ocean.
As we will discuss, the tides are the longest wavelength waves in the
entire ocean. Any submarine disturbance
powerful enough can cause a tsunami, such as a submarine landslide or a
submarine volcanic eruption. However,
the most likely way a tsunami forms is from a submarine earthquake. More precisely, the most likely way a tsunami
forms is from a submarine dip-slip earthquake.
As we discussed earlier in the course, a dip-slip fault is a vertical
break in rock, while a strike-slip fault is a horizontal break in rock. A submarine dip-slip earthquake causes a
vertical fault on the ocean floor. This
causes all of the water on top of the upward-thrusting rock to be thrust upward as well.
Also, all of the water on top of the
downward-thrusting rock falls downward.
These vertical motions create a tsunami, an ocean wave with an enormous
wavelength. The part of the ocean water
thrust upward is the initial crest of the tsunami, and the part of the ocean
water that falls downward is the initial trough of the tsunami. If a tsunami crest arrives at a continent
before a tsunami trough, then sea level will rise first, and the ocean will
first intrude into the continent. If a
tsunami trough arrives at a continent before a tsunami crest, then sea level
will drop first, and the ocean will first recede away from the continent. Regardless whether the crest or the trough
arrives first, a tsunami is a wave. So, there will be many alternating crests and troughs that
arrive to the continent. Every crest
causes sea level to rise, causing the ocean to intrude into the continent. Every trough causes sea level to drop,
causing the ocean to recede away from the continent. Again, any submarine disturbance powerful
enough could theoretically cause a tsunami, such as a submarine landslide, a
submarine volcanic eruption, and even a submarine strike-slip earthquake. However, the most likely way a tsunami forms
is from a submarine dip-slip earthquake.
Vertical motions of rock at the ocean floor are most likely to create
crests and troughs at the ocean surface.
Since the wavelength of tsunami is very long, even the deepest parts of
the ocean are shallow as far as tsunami are concerned. Therefore, a tsunami is born a shallow-water
wave. As the tsunami approaches the
continent, the depth of the ocean only becomes even more
shallow, and so the tsunami remains a shallow-water wave. Therefore, tsunami are always shallow-water
waves. Whereas most ocean waves
transition from deep-water waves to transitional waves to shallow-water waves,
tsunami do not transition from deep-water waves to transitional waves. Tsunami are born shallow-water waves, and
tsunami remain shallow-water waves. If
tsunami remain shallow-water waves as they approach a continent, then different
parts of a tsunami wave cannot catch up to each other to cause
interference. In
particular, there is no constructive interference, which means there is no
instability that would result in a breaker. In other words, tsunami do not break at the
continents as commonly depicted in disaster movies. As we will discuss, tides do not break
either. Both tsunami and tides are
always shallow-water waves that do not break.
Caution: this is the only thing that tsunami and tides share in common
with each other. The fact that they
share this in common is the reason why the old term
for tsunami was tidal waves.
Nevertheless, the term tidal wave is misleading, since it implies that
tsunami and tides are caused by the same
mechanism. This is false; tides are caused by a completely different process from submarine
disturbances, as we will discuss.
Therefore, we will always use the term tsunami, not tidal waves, to
emphasize that these waves are completely different from tides. The term tsunami is a Japanese word that
simply means harbor wave.
If a tsunami crest arrives at
a continent before a tsunami trough, then sea level will rise first, and the
ocean will first intrude into the continent.
This can kill thousands of people.
If a tsunami trough arrives at a continent before a tsunami crest, then
sea level will drop first, and the ocean will first recede away from the
continent. Ironically, even more people can be killed when this occurs. If the ocean first recedes away from the
continent, people who happen to be at the beach become intrigued and wander out
toward the receding ocean to explore the naked and exposed continental
shelf. Then, the first tsunami crest
arrives and kills all of those people in addition to people living inland when
the ocean intrudes into the continent.
If we ever witness the ocean recede away from the continent thus
exposing a naked continental shelf, we must leave the beach immediately. The first trough of a tsunami has just
arrived, and the first crest of the tsunami is on its way. Tsunami are rare in the Atlantic Ocean, since
the Atlantic Ocean is surrounded by passive
continental margins. However, tsunami
are common in the Pacific Ocean, since the Pacific Ocean is
surrounded by active continental margins, as we discussed earlier in the
course. Whenever seismologists trilaterate an earthquake to be somewhere at the bottom of
the Pacific Ocean, a tsunami warning is issued ordering everyone to leave the
shores and travel inland. This warning is issued throughout the west coast of South America, the
west coast of North America, the east coast of Asia, the east coast of
Australia, and even the Hawaiian Islands.
Unfortunately, there was no tsunami-warning system in the Indian Ocean
until more than two hundred thousand people were killed
from a tsunami in December 2004. After
this tragedy, a tsunami-warning system was established
in the Indian Ocean. Although tsunami
will continue to occur, hopefully such a tragic loss of human life as a result of a tsunami will now be avoided.
The tides are by far the
longest wavelength waves in the entire ocean.
We will reveal the wavelength of a tide shortly. All of us have some familiarity with the
tides. Sometimes it is high tide, more
correctly called the flood tide. Other
times it is low tide, more correctly called the ebb tide. The origin of the tides was
explained roughly three centuries ago by Isaac Newton, among the most
brilliant persons who have ever lived.
Newton discovered calculus (advanced mathematics) and invented physics
(the mathematical study of the equations that describe the universe) through
his discovery of three universal Laws of Motion and the law of Universal
Gravitation. According to Newton’s law
of Universal Gravitation, everything in the universe attracts everything else
in the universe. For example, the Earth
attracts the Moon gravitationally, and the Moon attracts the Earth
gravitationally. The Earth attracts the
Sun gravitationally, and the Sun attracts the Earth gravitationally. Isaac Newton proved mathematically that any
object exerts different gravitational attractions across any
other object due to the varying distances between different parts of the two
objects from each other. Parts of
the two objects that are closer to each other feel stronger gravitational
attractions, while parts of the two objects that are further from each other
feel weaker gravitational attractions.
The differences in the gravitational attractions across an object are called tidal forces, because they cause the tides in the
ocean. The Moon and the Sun each exert
roughly equal tidal forces on the Earth’s oceans causing them to bulge,
resulting in flood tides and ebb tides every day. When the Earth, the Moon, and the Sun happen
to form a nearly straight line (this occurs during New Moon or Full Moon), the
lunar tidal force and the solar tidal force reinforce each other. If we interpret the tides as waves, then
during this straight-line configuration, the lunar tidal crests and the solar
tidal crests meet each other, while the lunar tidal troughs and the solar tidal
troughs also meet each other. This is
constructive interference, resulting in severely high flood tides and severely
low ebb tides. These are
called the spring tides. When the
Earth, the Moon, and the Sun happen to form a nearly right angle (this occurs
during First Quarter Moon or Third Quarter Moon), the lunar tidal force and the
solar tidal force counteract each other.
If we interpret the tides as waves, then during this right-angle
configuration, the lunar tidal crests and the solar tidal troughs meet each
other, while the lunar tidal troughs and the solar tidal crests meet each
other. This is destructive interference,
resulting in modest flood tides and modest ebb tides. These are called the
neap tides. The tidal range is the wave
height of the tides, the difference between the flood tide and the ebb
tide. The spring tides have the largest
tidal range, since the flood tide is very high and the ebb tide is very low,
resulting in a large difference between them.
The neap tides have the smallest tidal range, since the flood tide is
modestly high and the ebb tide is modestly low, resulting in a small difference
between them. The Moon’s orbital period
around the Earth is roughly one month.
In fact, the word month is derived from the
word moon. If we take the word month,
remove the suffix -th, and
insert an extra letter o, we obtain the word moon! One month is roughly four weeks. If today is New Moon, we will have spring
tides with the largest tidal range (severely high flood tides and severely low
ebb tides). Roughly
one week later will be First Quarter Moon, and we will have neap tides with the
smallest tidal range (modest flood tides and modest ebb tides). Roughly one week
later will be Full Moon, and we will have spring tides again with the largest
tidal range (severely high flood tides and severely low ebb tides). Roughly one week
later will be Third Quarter Moon, and we will have neap tides again with the
smallest tidal range (modest flood tides and modest ebb tides). Roughly one week
later, we have returned to New Moon, roughly four weeks since the previous New
Moon. For thousands of years, humans
already noticed that there is a correlation between the changing appearance of
the Moon in the sky and the changing tidal range in the ocean, but it was Isaac
Newton who explained mathematically why this is the case. The lunar tidal force and the solar tidal
force not only cause the Earth’s oceans to bulge, but they also cause the shape
of the geosphere (the solid Earth) itself to bulge. The shape of the geosphere (the solid Earth)
itself suffers flood tides and ebb tides every day. When the geosphere (the solid Earth) itself
suffers a flood tide, we are slightly further from the center of the Earth. Later when the geosphere (the solid Earth)
itself suffers an ebb tide, we are slightly closer to the center of the
Earth. Each and every
day of our lives, we move up and down roughly one meter, even while we believe
ourselves to be remaining still!
Most coastlines experience
two unequal tides every day. In other
words, there are two flood tides with one higher than the other and two ebb
tides with one lower than the other.
This is called the mixed tidal pattern, the
most common tidal pattern. Some coastlines
experience two equal tides every day. In
other words, there are two flood tides that are equally high and two ebb tides
that are equally low. This is called the semidiurnal tidal pattern. The most rare tidal
pattern is the diurnal tidal pattern with only one flood tide and only one ebb
tide every day. These
different tidal patterns are caused by the configuration of the continents and
the shapes of coastal inlets such as bays and gulfs. Since the most common tidal pattern is two
tides every day, this means that the crest of a tide is on one side of planet
Earth, and the other crest of a tide is on the other side of planet Earth. Thus, the wavelength of a tide is half of the
circumference of planet Earth, since the wavelength of any wave is the distance
from one crest to the next crest, which is also the distance from one trough to
the next trough. The circumference of
planet Earth is roughly forty thousand kilometers. Therefore, the wavelength of a tide is
roughly twenty thousand kilometers! We
now understand that the tides are by far the longest wavelength waves in the
entire ocean. Since the wavelength of
the tides is so incredibly long, even the deepest parts of the ocean that are
only roughly eleven kilometers below sea level are extremely shallow as far as
the tides are concerned. Therefore,
tides are always shallow-water waves. As
a tide approaches the continent, the depth of the ocean only becomes even more shallow, and so the tide remains a shallow-water
wave. Whereas most ocean waves
transition from deep-water waves to transitional waves to shallow-water waves,
tides do not transition from deep-water waves to transitional waves. Tides are born shallow-water waves, and tides
remain shallow-water waves. If tides
remain shallow-water waves as they approach a continent, then different parts
of a tide cannot catch up to each other to cause interference. In particular, there is no
constructive interference, which means there is no instability that would
result in a breaker. In other
words, tides do not break. We know this
from common experience. Beginning with
the ebb tide, sea level simply gradually rises up to the flood tide, and then
sea level gradually drops down to the ebb tide.
This is the only thing that tides share in common with tsunami: neither
tides nor tsunami break since both tides and tsunami are always shallow-water
waves due to their extremely long wavelengths.
Tides may propagate up a
river causing flood tides and ebb tides upstream, not just at the shore. This is a wave that is
truly caused by the tides, and hence these waves should properly be
called tidal waves. Unfortunately, if we
refer to these waves as tidal waves, everyone will incorrectly assume that we
are referring to tsunami. Therefore, we
must give these waves a different name.
We will call a wave caused by the tides that propagates up a river a
tidal bore. These tidal bores are the
only waves that have the right to be correctly called
tidal waves.
The Bay of Fundy in Canada
between the Canadian provinces of Nova Scotia and New Brunswick experiences the
largest tidal range in the entire world.
This is because the shape of the Bay of Fundy happens to curve to the
right with the same curvature as the Coriolis force. This permits the Coriolis force to reinforce
the flood tide going into the bay every day.
During ebb tide, all of the water drains out of the bay lowering all
boats until they hit the bottom of the bay.
With the next flood tide, the entire bay floods, lifting all the boats off of the bottom of the bay. Any other bay in the northern hemisphere that
happened to curve to the right with the same curvature as the Coriolis force
would also suffer from an enormous tidal range.
If there happened to be a bay in the southern
hemisphere that happened to curve to the left with the same curvature as the
Coriolis force, it would also experience the same enormous tidal range as the
Bay of Fundy.
Transitional Landscapes/Environments: Coasts and Shores
All landscapes/environments
on planet Earth can be divided into three categories:
terrestrial landscapes/environments, marine landscapes/environments, and
transitional landscapes/environments.
Terrestrial landscapes/environments are on the continents. Marine landscapes/environments are at the
ocean floor. Transitional landscapes/environments are where
oceans and continents meet each other.
We have discussed marine landscapes/environments, and we will discuss
terrestrial landscapes/environments toward the end of the course. So, we now devote
some discussion to transitional landscapes/environments.
Transitional landscapes/environments are where
oceans and continents meet each other, commonly known as shores and
coasts. In colloquial English, the words
coast and shore are often used interchangeably. However, in oceanology/oceanography the words
coast and shore are given precise definitions. By these strict definitions, the coast and
the shore are actually two different things.
The shore is defined as where the continent and
the ocean meet each other. The coast is defined as the part of the continent inland that is
directly affected by the ocean. The
distance inland that the coast extends varies tremendously. Some coasts extend inland for dozens of
kilometers, while other coasts extend inland for less than one kilometer. The boundary between the coast and the shore is called the coastline.
The shore extends from the coastline into the ocean out to the edge of
the continental shelf. The precise
boundary where the continent and the ocean meet each other is
called the shoreline, but this shoreline moves every day due to the
tides. During the flood tide, the
shoreline is furthest inland, while during the ebb tide, the shoreline is
furthest from the continent. The
backshore is the part of the shore from the coastline to the flood-tide
shoreline. Therefore, the backshore is never submerged underwater, even during the flood
tide. The foreshore is the part of the
shore between the flood-tide shoreline and the ebb-tide shoreline. Sometimes the foreshore is
submerged, while at other times the foreshore is not submerged. The foreshore is completely
submerged during the flood tide, while the foreshore is completely above
sea level (completely not submerged) during the ebb tide. Most of the time, the foreshore is partially
submerged and partially not submerged.
The nearshore is the part of the shore from the ebb-tide shoreline to
the breakerline, where ocean waves first break as
surf. The offshore is the part of the
shore from the breakerline out to the edge of the
continental shelf. Both the nearshore
and the offshore are always completely submerged, even during the ebb
tide. The part of the shore humans
commonly call the beach is often (but not always) the foreshore.
Ocean waves that propagate
toward the continent will eventually splash onto the shore; this is called the swash.
The water then spills back into the ocean; this is called the
backwash. A backwash often has the next
swash splashing over it. Deep water
rapidly flowing away from the continent with overlying surface water rapidly
flowing toward the continent is called a rip
current. Humans who find themselves
within a rip current thus feel a double torque: deep water pulls their feet
away from the continent while surface water pushes their torso toward the
continent. Hence, humans standing within
a rip current are often knocked over and then pulled out into
the ocean. Many swimmers die
every year from rip currents. Attempting
to swim back toward the shoreline while being pulled
by a rip current expends energy, since we would be swimming against the rip
current. We must swim perpendicular to
the rip current, which is parallel or antiparallel to the shoreline, to successfully escape the rip current.
As an ocean wave approaches the
continent, the ocean wave slows due to the shoaling of the ocean. Refraction thus causes the ocean wave to bend
toward the normal; hence, the direction of the ocean wave becomes more
perpendicular to the shoreline.
Nevertheless, the direction of the ocean wave is still not perfectly
perpendicular to the shoreline. In other
words, the direction of the ocean wave can be broken into two components: one
component is perpendicular to the shoreline while the other component is along
(parallel or antiparallel to) the shoreline.
The component of an ocean wave along the shoreline is
called the longshore current, and this longshore current pushes sand
along the shoreline in the direction of the longshore current. Actually, each grain of sand executes a
zigzag trajectory as each swash pushes the sand grain toward the continent by a
small amount and then each backwash pulls the sand grain back into the ocean by
a small amount. Even if there were no
wind pushing the sand and even if there were no animals burrowing through the
sand and even if there were no humans building sandcastles, every grain of sand
at the beach is slowly but surely pushed along the shoreline in the direction
of the longshore current. It is for this
reason that oceanologists/oceanographers regard
beaches to literally be rivers of sand.
Natural forces continuously
destroy landscapes/environments on planet Earth; this is
called degradation. Natural
forces also continuously build new landscapes/environments on planet Earth;
this is called aggradation. Both degradation and aggradation almost always occur simultaneously. Nevertheless, a shore where there is much
more degradation with significantly less aggradation is called a degradational shore, while a shore where there is much more
aggradation with significantly less degradation is called an aggradational shore.
At degradational shores, ocean waves may carve
out a sea cave within the continent. Two
sea caves may merge, thus forming a sea arch over them. The sea arch may eventually collapse from its
own weight, leaving a sea stack in the ocean that is divorced from the
continent where there is now a sea cliff.
At aggradational shores, ocean waves may
create a long sand deposit that is curved on its
end. Butchers use long rods with hooks on
their ends in meat lockers; these long hooked rods used by butchers are called spits.
Since long sand deposits with curved ends at aggradational
shores resemble spits used by butchers, these long sand deposits with curved
ends at aggradational shores are
also called spits. The curved end
of an aggradational sand spit is
called a hook, such as Sandy Hook in New Jersey. The sand deposit may entirely cut off a
coastal inlet from the rest of the ocean, thus forming a bay barrier. Ocean waves may push sand to create a bridge
of sand that connects the continent to a sea stack; this is
called a tombolo. Entire islands
of sand parallel to the coastline may also be built at
aggradational shores; these are called barrier
islands, such as Long Beach Island in New Jersey. As some rivers approach the ocean, the river
water may deposit sand, forcing the river to break into many
distributaries. This aggradational
landform is called a delta.
Since the sea level actually
changes every day due to the tides, we should strictly use the term mean sea
level instead of sea level. The mean sea
level is the sea level averaged over many days, but even the mean sea level may
change tectonically, isostatically, or eustatically. A tectonic change in mean sea level is caused by a dip-slip motion of
the continent. If the continent
is thrust vertically upward, mean sea level relative
to that continent becomes lower. If the
continent is thrust vertically downward, mean sea
level relative to that continent becomes higher. An isostatic change in mean sea level results
from the buoyant motion of a tectonic plate.
If a tectonic plate sinks into the asthenosphere, mean sea level
relative to that continent becomes higher.
If a tectonic plate rises out of the asthenosphere, mean sea level
relative to that continent become lower.
Note that both tectonic and isostatic changes in mean sea level are
actually changes relative to the continent.
It is the continent that is actually moving; it
is not the ocean that is actually changing.
However, a eustatic change in mean sea level
is a global adjustment in mean sea level due to global climate change. For example, if planet Earth becomes colder
such as during a glacial period of an ice age, we expect mean sea level to
become lower for two reasons. Firstly,
the water that evaporates from the oceans precipitates onto the continent
forming enormous ice sheets that do not return to the ocean as rivers. Hence, water is subtracted
from the ocean without being added back to the ocean. Secondly, the water remaining in the ocean
thermally contracts due to the colder global temperatures. When the glacial period of an ice age ends,
we expect mean sea level to become higher for the same two reasons. Firstly, the enormous ice sheets that cover
the continents melt into liquid water that returns to the ocean, thus
replenishing the water that was formerly subtracted. Secondly, the water in the ocean thermally
expands due to the warmer global temperatures.
Note that isostatic and eustatic changes in
mean sea level compensate for one another, resulting in only small changes in
the mean sea level. For example, if
planet Earth becomes colder such as during a glacial period of an ice age, we
have argued that there will be a eustatic drop in
mean sea level. However, the enormous
ice sheets that form upon the continents will push the continental plates into
the asthenosphere, and hence there will be an isostatic rise in mean sea level
relative to the sinking continents! The eustatic drop in mean sea level is compensated
by the isostatic rise in mean sea level relative to the sinking continental
plates. More plainly, continents and
oceans both drop together, and hence the actual change in mean sea level is
small. Conversely, if planet Earth
becomes warmer such as at the end of a glacial period of an ice age, we have
argued that there will be a eustatic rise in mean sea
level. However, the melting ice sheets
remove weight off of the continents permitting the
continental plates to rise out of the asthenosphere, and hence there will be an
isostatic drop in mean sea level relative to the rising continents! The eustatic rise in mean sea level is compensated by the isostatic drop in
mean sea level relative to the rising continental plates. More plainly, continents and oceans both rise
together, and hence the actual change in mean sea level is again small.
Whether the mean sea level
changes tectonically, isostatically, eustatically, or some combination thereof, we can classify shores as either emergent shores or submergent
shores based upon the change in mean sea level.
If the mean sea level is dropping, then we call the shore an emergent
shore. If the mean sea level is rising,
then we call the shore a submergent shore. Entire beaches can be
drowned in submergent shores. A drowned river valley in a submergent shore results in estuaries where the mouth of a
river opens wide to meet the ocean.
Ocean water has a higher salinity than river water. As a result, ocean water is more dense, while river water is less dense. At an estuary, the more
dense ocean water sinks, while the less dense river water rises. Therefore, the limnological
river water sits on top of the seawater at estuaries. Between the more dense
deep ocean water and the less dense surface river water, the seawater mixes
with the limnological water to form brackish water.
Oil spills are
commonly considered a form of ocean pollution, but this is false. Firstly, liquid petroleum is
naturally leaked into the ocean from fissures on the ocean floor. In fact, the total amount of oil naturally
leaked into the ocean from fissures every year is roughly double the total
amount of oil spilled into the ocean by oil tankers every year. Since petroleum is a hydrocarbon, petroleum that leaks into the ocean is consumed by the plankton
that live in the ocean. The
plankton then proliferate, the small fish that eat the plankton thus
proliferate, and so on and so forth. In
summary, the leaking of petroleum into the oceans enhances marine biological
productivity. Human-caused oil spills
are not an exception; after all, petroleum is petroleum! As evidence of the benefits of oil spills to
marine life, let us discuss the Exxon Valdez oil spill as a case study. In March 1989, the oil tanker Exxon Valdez
accidentally collided with Bligh Island Reef and spilled between eleven million
and twelve million gallons of oil into Prince William Sound in Alaska. As a result of this
oil spill, the ecology of Prince William Sound was enhanced in numerous
ways. Firstly, the population of the
cormorant (an aquatic bird) was in decline before the spill, but the population
of the cormorant doubled over the next decade immediately after the spill! Secondly, the population of the bald eagle was also positively affected. The bald eagle is the national bird of the
United States. The bald eagle was an
endangered species several decades ago; it would be a disgrace if the national
bird of the United States were to become extinct. Fortunately, the population of the bald eagle
increased so much over the several years immediately after the Exxon Valdez oil
spill that the United States Fish and Wildlife Service removed it from the
endangered species list as a direct result of the spill! Thirdly, the population of the harbor seal
was declining by roughly twelve percent every year before the spill, but then
the population of the harbor seal declined by only six percent every year over
the next several years after the spill.
If more oil was spilled, perhaps the population
of the harbor seal would have increased!
Fourthly, Exxon spent more than two billion dollars immediately after
the spill and an additional nine hundred million dollars over the next few
years cleaning the spilled oil. However,
oceanological/oceanographic surveys of Prince William
Sound over the next decade showed that the areas not cleaned by Exxon thrived ecologically while the areas that were cleaned by
Exxon did not thrive ecologically! In
other words, Exxon should not have cleaned any of the oil that was spilled! The
population of the pacific herring remained unchanged for a few years after the
spill. Then, the population suddenly
dropped by roughly eighty percent. Is
this a negative consequence of the Exxon Valdez oil spill? No, since most oceanologists/oceanographers
agree that this was caused by disease unrelated to the
spill. Indeed, the population of the
pacific herring slowly increased after the disease ended. To summarize, oil spills should
not be considered a form of pollution, since oil spills are actually
beneficial to marine ecology. Instead of
punishing oil companies that accidentally spill oil into the ocean, governments
should reward oil companies that spill oil into the ocean for enhancing the
marine ecology of our planet Earth.
copyeditor: Michael Brzostek (Spring2023)
Links
Libarid A. Maljian homepage at the Department of Physics at CSLA at NJIT
Libarid A. Maljian profile at the Department of Physics at CSLA at NJIT
Department of Physics at CSLA at NJIT
College of Science and Liberal Arts at NJIT
New Jersey Institute of Technology
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